Fight involving energy weapon (5)
I believe the answer is:
spear
'weapon' is the definition.
(spear is a kind of weapon)
'fight involving energy' is the wordplay.
'fight' becomes 'spar' (sparring is a kind of fighting**).
'involving' is an insertion indicator.
'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 "stick" , "Spike" , "Asparagus shoot" , "Stabbing, throwing, weapon" , "Shafted weapon for throwing or thrusting" .)