Fight a big-headed brute (3)
I believe the answer is:
box
'fight' is the definition.
(boxing is a kind of fighting)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for box that I've seen before include "Punch" , "Spar; tree" , "Engage in pugilism" , "Topiary shrub" , "Receptacle" .)