Trout swimming round black fish (6)
I believe the answer is:
turbot
'fish' is the definition.
(turbot is a kind of fish)
'trout swimming round black' is the wordplay.
'swimming' is an anagram indicator.
'round' is an insertion indicator.
'black' becomes 'b' (as in pencil classifications).
'trout' with letters rearranged gives 'turot'.
'turot' placed around 'b' is 'TURBOT'.
(Other definitions for turbot that I've seen before include "Expensive flatfish" , "Flatfish - trout B (anag)" , "Large food fish" , "Pale brown, speckled flatfish" , "Large succulent flatfish" .)