Coward went into melee punching heads (4)
I believe the answer is:
wimp
'coward' is the definition.
(both can mean a weak or frightened person)
'went into melee punching heads' is the wordplay.
'heads' suggests taking the first letters.
The first letters of 'went into melee punching' is 'WIMP'.
(Other definitions for wimp that I've seen before include "Feeble individual" , "Feeble or ineffective man" , "Mummy's boy" , "Feeble or ineffective person" , "A weak ineffectual man" .)