Ah, thanks Quisling. I saw that wordplay earlier but didn’t know the meaning. It works as a plural noun as well I think, if Collins online is our reference.
Thanks Quisling for the brilliant parsing as usual. Could zero in on the word. I was trying to analyse mental (slang word for bats) containg N A for North American and was getting nowhere near the answer. Your cue finally helped me to solve the clue.
4-letter hollow inside 3-letter 'which people' (as a question) + a single letter from a word in the clue
Definition is completely, and the answer is the last two words of what is normally a four-word expression