Capsizing in warfare, weakened vessel (4)
I believe the answer is:
ewer
'vessel' is the definition.
(ewer is a type of jug)
'capsizing in warfare weakened' is the wordplay.
'capsizing' shows that the letters should be reversed in order.
'in' indicates a hidden word.
'rewe' can be found hidden inside 'warfare weakened'.
'rewe' in reverse letter order is 'EWER'.
(Other definitions for ewer that I've seen before include "Water jug with wide mouth" , "Large pitcher" , "29 [VESSEL]" , "A type of jug" , "Pitcher, water jug" .)