Be of use (5)

I think it could be either:

serve

avail

'be of use' is the definition.
(I've seen this before)

This is the entire clue.

(Other definitions for avail that I've seen before include "Make use, take advantage" , "(Of no) use or benefit" , "Provide help; profit" , "Medal (informal)" , "Help, benefit" .)

I've seen this clue in The Telegraph, the King Feature Syndicate, The Guardian, the Evening Standard, The New York Times, The Mirror, the Universal, the LA Times, The Independent and the Sydney Morning Herald.

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