Thanks all found it now (and I already
had 21 ac). Curiously, the one I had missed had a perfectly reasonable (tho very obscure) parsing of the clue which fitted in without modification!
I've solved all the clues and I understand the common feature in the 11 "special" ones. However, if I remove the common feature I end up with an answer that's always one letter too short. A hint as to how to treat these would be very welcome!
Not easy. I worked out what the "operation" was fairly early, but couldn't find out where the "operation's" definition was. I now have 9A, but how does it parse? And any help with the only other one left now, 27A, would be gratefully received.