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Hi everyone, thanks for all the comments - I left this one alone today and have just returned to it to find an answer which satisfies a reading of the instruction. I'm not convinced 20ac has more than one possible solution: I have three letters that fit but only one which satisfies the cryptic.
Still, I'm afraid, deeply underwhelmed: I would not be particularly surprised if Green Lanes received a variety of "answers" as there's nothing particular to check your answer against. It is, in effect, a kind of cryptic clue,with nothing to indicate length of solution or format, or to cross check the accuracy which seems to be needed.
If considered as a the run up to a phrase or word (ie if you put .... at the end) the instruction could lead to, well, anything, as I suggested earlier, and "anything" and lots of other entries could be argued for.
Unless I'm still missing something, there's no real connection between the puzzle per se and the "answer" - there's no development of a theme, it's just tacked on as a rather separate tailpiece. Sorry guy and gals - and Trev - I just didn't like the lack of cohesion. Tell me I'm wrong!