I thought this was going to be easy when I saw the "way in" straight away and a whole bunch of solutions fell into place; in addition, a lot of posts are reporting completed puzzles.
Now I find myself bogged down even more than normal...
I never imagined I'd be saying this about a numeric Listener, but I'm having trouble parsing the clue to 17 ac rather than the answer!
(I+T-I) obviously equates to T, but is the outer bracket meant to be interpreted as "(T*G)+A" or (T*(G+A)"? My spreadsheet software can't cope with the given combo.