Capsize plain vessel (8)
I believe the answer is:
overturn
'capsize' is the definition.
(I've seen this before)
'plain vessel' is the wordplay.
'plain' becomes 'overt' (both can mean visible or obvious).
'vessel' becomes 'urn' (urn is a type of container).
'overt'+'urn'='OVERTURN'
(Other definitions for overturn that I've seen before include "Upset; abolish" , "Reverse, rescind," , "tip" , "Reverse (legal ruling)" , "Invert" .)