Fight using weapon heartlessly (4)
I believe the answer is:
spar
'fight' is the definition.
(I know that fight is a is a more specific form of the action of spar)
'weapon heartlessly' is the wordplay.
'weapon' becomes 'spear' (spear is a kind of weapon).
'heartlessly' suggests removing the centre.
'spear' with its middle removed is 'SPAR'.
'using' acts as a link.
(Other definitions for spar that I've seen before include "Translucent mineral" , "Box; mineral; pole" , "Mineral; plane's wing beam" , "quarrel" , "Pole used in rigging - fight" .)