Burnt the fish - a kind of mullet (7)
I believe the answer is:
charred
'burnt' is the definition.
(char can mean to burn or blacken)
'fish a kind of mullet' is the wordplay.
'fish' becomes 'char' (char is a kind of fish).
'a kind of mullet' becomes 'red'.
'char'+'red'='CHARRED'
'the' is the link.
(Other definitions for charred that I've seen before include "Partially burned" , "Blackened by fire" , "Slightly burnt" , "Partially burnt" , "what's black on surface" .)