Expert swimmer (3)

I believe the answer is:

dab

This clue is a double definition.

'expert' is the first definition.
(thesaurus)

'swimmer' is the second definition.
(I've seen this before)

(Other definitions for dab that I've seen before include "Fish; small amount" , "Apply light touches of colour to the fish" , "Bad kind of fish" , "Digital radio" , "Small North Atlantic flatfish" .)

I've seen this clue in the Evening Standard.

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