Note: You will probably want some form of graph paper, square paper, crossword-constructing program, spreadsheet software, or some tool for inputting things into a grid to solve this. A blank 25x25 grid has been provided for convenience, but this does not imply that the size of the grid is 25x25; the size of the grid is for the solver to determine.
The answer is a single word, thematic to the puzzle.
Many thanks to Deusovi, mathgrant, Mad Jaqk, Projectyl, and ExtraTricky for testsolving this.
ERRATUM: At 19/04/2018, 08:35, the clues 17A, 34A and 1D were amended, as an earlier version of the puzzle was uploaded by accident. Your progress should not have been affected.
1 Make dark mountain hold deficiency
4 Environment Secretary meeting with nurse, or politician?
7 Save money for a radio
9 Papers for chart in steel amalgamate
10 I taunt lad; it breaks down, feeling related
11 Enrage badly, without end!
12 Boba to wiretap iocaine smuggler
14 Laved due, unfortunately, went down in price
15 Froth from sclera fluid
17 Poetry menace Cyril
18 Opening found in print roller
20 Reminder of right speed, in hindsight
22 Snitches, captured: point values?
24 Later, behind protections
26 Repairs what we hear mongrel destroys
27 Chaos from left changing direction, amidst yen collapse
29 Hero-destroyed clothing: wear it
30 For example, Snapchat collage? Ideal mosaic! (2 wds)
32 Incorrectly label ricin as "glitter"
34 Outlines of pruned shrubs
35 Repeating without one answering
36 Tool on German submarine? Silent? Worrisome
1 Two nibbles by the heartless
2 Speed up treacle ace? Difficult
3 Student ran tirelessly, hid contestant
4 Germanic people, rating, shot wildly
5 Unpredictably, Eric ate rodent companion
6 Circuit's gate with some data
7 King or Queen in mix-up, slain? That's murder, for one (2 wds)
8 Foreshadowed queen of the gods led five-hundred within
11 Newer hay, tossed around, is in every place
13 Phone a friend who's a clown, maybe?
14 Sensitive operator #99 had lunch, for example
16 Draws anti-tank plots
19 The Reverend reports a patchwork car near the pavement
21 Understands rises
23 Basically, the Spanish sent me all year
24 Chocolate bar's ultra-erotic contents
25 Large pipes swat airmen in battle (2 wds)
26 Comforted criminal put bottom on shoe
28 Cry loudly, "Why, behead the moral traitor!" (2 wds)
31 Length of weather office's reply
32 Radical type's second organization?
33 Prison square