Many an expert swimmer (4)
I believe the answer is:
dace
'swimmer' is the definition.
(dace is a kind of fish)
'many an expert' is the wordplay.
'many' becomes 'd' (Roman numeral for 500).
'an expert' becomes 'ace' (ace can informally mean very good).
'd'+'ace'='DACE'
(Other definitions for dace that I've seen before include "Small river fish" , "Freshwater fish" , "Fish in the carp family" , "Chubby swimmer" .)