Big guns getting sick boarding vessel (9)
I believe the answer is:
artillery
'big guns' is the definition.
(I've seen this before)
'sick boarding vessel' is the wordplay.
'sick' becomes 'ill' ('ill' can be a synonym of 'sick').
'boarding' indicates putting letters inside.
'vessel' becomes 'artery' (artery is a kind of vessel**).
'ill' going inside 'artery' is 'ARTILLERY'.
'getting' is the link.
(Other definitions for artillery that I've seen before include "Regiment" , "Large-calibre guns" , "Heavy weapons" , "Large pieces" , "Army unit that uses big guns" .)