A Riddle Like a Phoenix

Set right within a wood, you see
An army group with energy.
Addendum in the heart may be
Guffawing inappropriately.
Behold! O sad remains the key
That holds the ace front backwardly

The answer is a single English word.

• This is a beautiful puzzle! I think it could be improved slightly by putting the first line in iambic tetrameter (which is the meter of all the other lines). All you'd have to do is to add an initial "O", as in "O right within a wood you see". Jan 18, 2017 at 16:01
• @Silenus I hear you. I considered it but I used that cheat on the 5th line where I felt I couldn't do without it. I'm not sure if it's worse to fail to scan or to pad twice with "O". The other option is to start with the word "Set". I can't remember why I didn't go for that. Jan 18, 2017 at 19:04
• Oh yes, "set" works nicely... Jan 18, 2017 at 19:08

CORPSE.

Right within a wood you see

Right (R) within a wood (COPSE).

An army group with energy.

An army group (CORPS) with energy (E).

Addendum in the heart may be

Addendum (PS) in the heart (CORE) - thanks to @Deusovi for catching this.

Guffawing inappropriately.

Corpsing, for an actor, means to guffaw inappropriately.

Behold! O sad remains the key
That holds the ace front backwardly

Pretty sure this is another piece of wordplay - perhaps something to do with the letter O, or "ace front" meaning A? Or as Volatility suggests, behold -> see -> C, then O, then the rest somehow cluing RPSE?

OP edit: Putting together the last pieces. On the last two lines I attempted (perhaps too successfully) to misdirect the user with punctuation and enjambment (line breaks). These are actually two clues with the split between "remains" and "the key". The first clue is a definition. It becomes "Behold: o sad remains!" or "Lookit the corpse!" The second clue is "The key that holds the ace front backwardly." This refers to the escape key, ESC, holding PRO, a synonym for a skilled person or ace. This gives "ES(PRO)C". Taken "front backwardly": "C(ORP)SE". The title is a reference to the way a phoenix rises from the ashes of its former self; that is, from a corpse. My private joke is that I made this puzzle as an indirect hint for this puzzle, making the title doubly apt. And, yes, I do realize this makes me an insufferable meta-geek.

• My best guess for the last two lines is something like "Behold" -> "See" -> C, O, "sad remains" somehow -> S or "sad" -> S and "remains" cluing the actual word, "the key" -> E, containing "ace front backwardly" -> "professional front backwardly" -> PR backward -> RP, for C+O(RP<)S+E. Jan 18, 2017 at 0:21
• For the last one, 'the key that holds the ace' is space (spacebar), front backwardly is ps, so put that together with RE from remain and you get RPSE
– jdow
Jan 18, 2017 at 2:07
• @jdow Good thinking! Different key. Jan 18, 2017 at 4:54
• The last two lines are tricky. Enjambement. Punctuation. Jan 18, 2017 at 4:55
• @Khale_Kitha Not to diminish the noble backspace, I had yet another key in mind. Jan 18, 2017 at 16:31