Fighting power that's corrupt (4)
I believe the answer is:
warp
'corrupt' is the definition.
(thesaurus)
'fighting power' is the wordplay.
'fighting' becomes 'war'.
'power' becomes 'p'.
'war'+'p'='WARP'
'that's' acts as a link.
(Other definitions for warp that I've seen before include "Distort as wood may from heat" , "Pervert - distort" , "Abnormality in character" , "Go out of shape" , "Become twisted" .)