Really enjoyed this thanks, some great clues. My answers take the form of 'ANSWER - definition, explanation of wordplay' with stuff in square brackets containing extra comments.
[Edit: Having seen DevOfZot's answer, I would say that 7D may be too tricky for an indirect anagram, whereas 21D is acceptible. Of course, YMMV!]
Across
- Not-all-there sibling touches bit of leg in lewd locale (7)
BROTHEL - Lewd locale, BROTHE (sibling [brother] not all there) + L (bit of leg)
- Discussion about number one steamship adds "it is Spanish" (7)
SESSION - Discussion, NO (number) + I (one) + SS (steamship) + ES (Spanish 'it is') backwards (about)
- Give directions before the writer? (9)
PRESCRIBE - Give directions, PRE (before) + SCRIBE (the writer)
- Permit to call ownership of holdings (5)
ALLOW - Permit, contained in (holdings) cALL OWnership
- Be aware; it sounds like a melody (6)
ATTUNE - Become aware, sounds like A TUNE (a melody)
- Mold takes head of white snake into small vessel (7)
ROWBOAT - Small vessel, W (head of white) + BOA (snake) inside ROT (mold takes)
- To jog backwards is a crime (4)
TORT - A crime, TROT (jog) backwards
- Most of shattered lantern found in back section of undeveloped area (10)
HINTERLAND - undeveloped area, anagram of (shattered) LANTER (most of lantern) in HIND (back section)
- Born anew, Fang led his pack into the embraces of present-day (10)
NEW FANGLED - present-day, contained in (into the embraces of) born aNEW FANG LED his pack
- City will not gerrymander (4)
TOWN - City, anagram of (gerrymander [meaning to move political boundaries]) WON'T (will not) [loved the use of gerrymander as anagram indicator - not sure I've seen it used before. The clue reads very well]
- Ice cast away by monk (7)
ASCETIC - A monk lives an ascetic life, anagram of (away) ICE CAST
- "Yessir", I stuttered in the auditorium (3,3)
AYE AYE - Yessir, sounds like (in the auditorium) I I (I stuttered) [Really like this one]
- Canoe flipped at sea (5)
OCEAN - Sea, anagram of (flipped) CANOE
- Upside-down whale takes false symbolic representation (9)
ACROPHONY - Symbolic representation [when the names of letters in an alphabet begin with the letters themselves], ORCA backwards (upside-down whale) + PHONY (false) [I had to look up the definition - a new one on me!] [A purist might question the use of 'upside-down' in an across clue]
- Putting bit of raw rat into boiled emu is a mistake (7)
ERRATUM - A mistake [in writing], R (bit of raw) RAT inside (putting into) anagram of (boiled) EMU
- Squashes secondhand kid (7)
STEPSON - Kid, STEPS ON (squashes)
Down
- Beginning of broad assault potentially threatens short Protestant (4)
BAPT - Protestant, Beginnings of Broad Assault Potentially Threatens [short for Baptist? Is it actually a word or is it an abbreviation?]
- Shoot past and conquer (9)
OVERTHROW - Conquer, OVER THROW (shoot past)
- Spasm in strange part of Macchu Picchu (6)
HICCUP - Spasm, anagram of (strange) PICCHU (part of Macchu Picchu)
- Moving slowly, I tire (long walks) (9)
LOITERING - Moving slowly, anagram of (walks) I TIRE LONG
- Incorrectly parse weapon (5)
SPEAR - Weapon, anagram of (incorrectly) PARSE
- Loyalist broke law in the beginning (8)
STALWART - Loyalist, anagram of (broke) LAW in START (the beginning)
- Mix bit of grass with scattered potpourri in cool house (5)
IGLOO - Cool house, G (bit of grass) inside (mix) anagram of (scattered) OLIO (potpourri) [tough one - I vaguely think I might have seen OLIO before in a crossword, but I had to look up the definition]
- Early learner is stunned, wet, confused (3,7)
NEW STUDENT - Early learner, Anagram of (confused) WET STUNNED
- Self-contained way to not sit by yourself? (10)
STANDALONE - Self-contained, STAND (not sit) ALONE (by yourself)
- Perplexed user shut a reference book (9)
THESAURUS - Reference book, Anagram of (perplexed) USER SHUT A
- Nameless young man's son endlessly beaten (9)
ANONYMOUS - Nameless, anagram of (beaten) YOUN MAN SO (young man's endlessly)
- Percussion instrument is a way to catch fish (8)
CASTANET - Percussion instrument, CAST A NET (way to catch fish) [nice wordplay!]
- Unusual pontiff heads limited edition community (6)
PEOPLE - Community, anagram of (unusual) POPE (pontiff) + LE (limited edition)
- Sharp left in confusing race (5)
CLEAR - Sharp (e.g. in sharp / clear focus), L (left) in anagram of (confusing) RACE
- Where was I? Inside a mollusk, I swear (5)
CLAIM - swear (to claim or swear something), I inside CLAM (mollusc)
- He hears a song (4)
HYMN - A song, sounds like HIM (he)