We’re told to identify the fish (6)
I believe the answer is:
plaice
'the fish' is the definition.
(plaice is a kind of fish)
'we're told to identify' is the wordplay.
'we're told' indicates a 'sounds like' (homophone) clue.
'to identify' becomes 'place' ('place' can be a synonym of 'identify').
'place' sounds like 'PLAICE'.
(Other definitions for plaice that I've seen before include "Large flatfish" , "Edible flatfish" , "Brown flatfish" , "A fish; sounds like put" , "Flat fare" .)