Topple open vessel (8)
I believe the answer is:
overturn
'topple' is the definition.
(I've seen this in another clue)
'open vessel' is the wordplay.
'open' becomes 'overt' (synonyms).
'vessel' becomes 'urn' (urn is a kind of vessel).
'overt'+'urn'='OVERTURN'
(Other definitions for overturn that I've seen before include "Reverse, rescind," , "Invert" , "Reverse (decision)" , "Invalidate - upset" , "Reverse previous findings" .)