Eighteen clues contain deficient wordplay, the missing elements of which, when highlighted, confirm the theme.
 

Across

1 Party with a previously haughty man (6)
6 Waste disposal initially declined (7)
11 Small braces (4)
12 Dog has a drink outside well (5, 4)
13 Boozer hugging a member’s tree (5, 3)
14 Old coins from the Parisian musical (6)
15 Bloody fool’s excellent! (5)
16 Plant box Alison covers (6)
18 Cash error — not good (3)
20 Officer deserting error-free break in (6)
21 High-tension Bletchley Park unit (3, 3)
22 Paint a terribly venomous snake (6)
23 Fast-moving quiet echinoderm (8)
29 Add to layer (8)
32 Dullard beat a groin (6)
33 Piece of...