Buckle quietly after fighting (4)
I believe the answer is:
warp
'buckle' is the definition.
('warp' can be a synonym of 'buckle')
'quietly after fighting' is the wordplay.
'quietly' becomes 'p' (p for piano and quiet).
'after' means one lot of letters go next to another.
'fighting' becomes 'war'.
'p' put after 'war' is 'WARP'.
(Other definitions for warp that I've seen before include "Make or become out of shape" , "Pervert - distort" , "Crane fly larva" , "Of wood, twist under heat" , "Distort as wood may from heat" .)