Burn the fish! (4)
I believe the answer is:
char
I believe this is a double definition.
'burn' is the first definition.
(charring is a kind of burning)
'the fish' is the second definition.
(char is a kind of fish)
(Other definitions for char that I've seen before include "Singe, scorch" , "Overcook - fish - daily" , "Daily cleaner; burn" , "Person who cleans" , "Burn; cleaner; fish" .)