Trout swimming around black fish (6)
I believe the answer is:
turbot
'fish' is the definition.
(type of fish)
'trout swimming around black' is the wordplay.
'swimming' is an anagram indicator (letters swim around into a new order).
'around' is an insertion indicator.
'black' becomes 'b' (abbreviation 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 "Species of fish" , "[SOMETHING FISHY]" , "Edible flatfish" , "Large succulent flatfish" , "Pale brown, speckled flatfish" .)