Sort of trout swallowing Black Sea fish (6)
I believe the answer is:
turbot
'sea fish' is the definition.
(turbot is a kind of sea fish)
'sort of trout swallowing black' is the wordplay.
'sort of' indicates anagramming the letters.
'swallowing' indicates putting letters inside.
'black' becomes 'b' (as in pencil classifications).
'trout' with letters rearranged gives 'turot'.
'turot' going around 'b' is 'TURBOT'.
(Other definitions for turbot that I've seen before include "Edible flatfish" , "Swimmer" , "Large brownish European flatfish" , "Rub tot out with the fish" , "Type of fish" .)