Roman's capsized sailor's vessel (6)
I believe the answer is:
mortar
'vessel' is the definition.
(mortar is a kind of vessel)
'roman's capsized sailor's' is the wordplay.
'roman' becomes 'rom'.
'capsized' shows that the letters should be reversed in order.
'sailor' becomes 'tar' (tar is a kind of sailor).
'rom' written backwards gives 'mor'.
'mor'+'tar'='MORTAR'
(Other definitions for mortar that I've seen before include "Artillery weapon - building mixture - vessel with a pestle" , "Gun; building material" , "Bomb-firing weapon" , "A short cannon for firing shells" , "Lime mixture; a cannon" .)