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' shows that the letters should be reversed in order.
'with' says to put letters next to each other.
'nut' backwards 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" .)