Late to the party as ever (our plan to work on the puzzle during Sat lunch on sunny pub terrace was thwarted when the found the newsagent en route had sold out of Guardians, so had a Weekend I with our lunch and had to wait to print the puzzle when we got home. Pub has no wifi.)
Finished, to my surprise. Gave up and came to the forum with only 9 solved (FOI 14a), but thanks to a1@11, birder@22 and johnny@28 we got the big 7,1 and then the clever 23, and raced on from there. Returning to read the rest of the forum explained some mystery parsings, and the discussion of subtractive or compound anagrams resolved 18, where I had alternative letters "l" and "s" for the first 2 uncrossed letters giving a just-about plausible definition!
Got in a muddle, delaying solving 1a, by absentmindedly writing first word of wrong US govt body (it crossed Ok with 14, and didn't delay solving 12, but I couldn't solve 1a ending in "c"!)
Thanks for your help, all. A tough but entertaining puzzle.