Bachelor tucking into cooked trout — or flatfish (6)
I believe the answer is:
turbot
'flatfish' is the definition.
(turbot is a kind of flatfish)
'bachelor tucking into cooked trout' is the wordplay.
'bachelor' becomes 'B' (abbreviation e.g. BA=Bachelor of Arts).
'tucking into' is an insertion indicator.
'cooked' indicates anagramming the letters (letters cooked into a new form).
'trout' with letters rearranged gives 'turot'.
'b' placed within 'turot' is 'TURBOT'.
'or' acts as a link.
(Other definitions for turbot that I've seen before include "Large food fish" , "Expensive flatfish" , "Rub tot out with the fish" , "Swimmer" , "Large brownish European flatfish" .)