Trout swimming around black fish (6)
I believe the answer is:
turbot
'fish' is the definition.
(turbot is a kind of fish)
'trout swimming around black' is the wordplay.
'swimming' indicates anagramming the letters.
'around' is an insertion indicator.
'black' becomes 'b' (as in pencil classifications).
'trout' is an anagram of 'turot'.
'turot' enclosing '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" .)