Head back with a fish (4)
I believe the answer is:
tuna
'fish' is the definition.
(tuna is a type of fish)
'head back with a' is the wordplay.
'head' becomes 'nut' (informal term for somebody's head).
'back' says the letters should be written backwards.
'with' is a charade indicator (letters next to each other).
'nut' in reverse letter order is 'tun'.
'tun'+'a'='TUNA'
(Other definitions for tuna that I've seen before include "Large sea-fish" , "Tunny" , "A fish; sounds like piano juster" , "Large salt water fish" , "Large edible sea fish, often canned" .)