Strikes back in skirmish (4)
I believe the answer is:
spar
'skirmish' is the definition.
(thesaurus)
'strikes back' is the wordplay.
'strikes' becomes 'raps' (rapping is a kind of striking).
'back' says the letters should be written backwards.
'raps' written backwards gives 'SPAR'.
'in' acts as a link.
(Other definitions for spar that I've seen before include "Train at boxing; mineral" , "Raps with wooden beam" , "Length of wood" , "dispute" , "Box - girder" .)