Open vessel to capsize (8)
I believe the answer is:
overturn
'capsize' is the definition.
(thesaurus)
'open vessel' is the wordplay.
'open' becomes 'overt' ('overt' is associated in meaning with 'open').
'vessel' becomes 'urn' (urn is a kind of vessel).
'overt'+'urn'='OVERTURN'
'to' acts as a link.
(Other definitions for overturn that I've seen before include "Invalidate - upset" , "Revoke" , "Reverse; upend" , "Reverse (legal ruling)" , "Reverse or capsize" .)