Big guns suffering boarding vessel (9)
I believe the answer is:
artillery
'big guns' is the definition.
(I've seen this before)
'suffering boarding vessel' is the wordplay.
'suffering' becomes 'ill'.
'boarding' means one lot of letters goes inside another.
'vessel' becomes 'artery' (artery is a kind of vessel**).
'ill' going within 'artery' is 'ARTILLERY'.
(Other definitions for artillery that I've seen before include "Gunners" , "Large pieces" , "Powerful guns" , "group of soldiers" , "Large guns for use on land" .)