It's only synonyms of the themed clues that are entered, so can't give letters.
31a. Pigeon that's used to release a bolt (7). The synonym is 5 letters and is probably G-A-S
I am guessing the G is from CAGE which I presume is a synonym of enclosure. Sorry to those who haven't got the actual crossword. I find Brummie's added complexity in the seven themed clues, somewhat irritating/ not witty.