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