Fight about energy weapon (5)
I believe the answer is:
spear
'weapon' is the definition.
(spear is a kind of weapon)
'fight about energy' is the wordplay.
'fight' becomes 'spar' (sparring is a kind of fighting**).
'about' indicates putting letters inside.
'energy' becomes 'e' (physics symbol as in E=mcp).
'spar' placed around 'e' is 'SPEAR'.
(Other definitions for spear that I've seen before include "mint may be added" , "A bit of asparagus" , "asparagus for example" , "Shafted weapon for throwing or thrusting" , "Sharp weapon" .)