Fish in burn? (4)
I believe the answer is:
char
I believe this clue is a double definition.
'fish' is the first definition.
(char is a kind of fish)
'burn?' is the second definition.
(charring is a kind of burning)
'in' acts as a link.
(Other definitions for char that I've seen before include "Daily cleaner; burn" , "'Mrs Mopp'" , "Become black by burning" , "Cleaner - tea" , "tea > daily" .)