Fight a bighead with beef! (3)
I believe the answer is:
box
'fight' is the definition.
(boxing is a kind of fighting)
'a bighead with beef' is the wordplay.
'a bighead' becomes 'b' (the head letter of 'big').
'with' is a charade indicator (letters next to each other).
'beef' becomes 'ox'.
'b'+'ox'='BOX'
(Other definitions for box that I've seen before include "Wooden case" , "Present container?" , "Punch" , "Fight; tree; container" , "Christmas present" .)