Battery ailing in vessel (9)
I believe the answer is:
artillery
'battery' is the definition.
(thesaurus)
'ailing in vessel' is the wordplay.
'ailing' becomes 'ill' ('ill' can be similar in meaning to 'ailing').
'in' is an insertion indicator.
'vessel' becomes 'artery' (artery is a kind of vessel**).
'ill' put inside 'artery' is 'ARTILLERY'.
(Other definitions for artillery that I've seen before include "Regiment" , "group of soldiers" , "Army unit that uses big guns" , "Large pieces" , "Heavy weapons" .)