Upset open vessel (8)
I believe the answer is:
overturn
'upset' is the definition.
(I know that upset can be written as overturn)
'open vessel' is the wordplay.
'open' becomes 'overt' (associated in meaning).
'vessel' becomes 'urn' (urn is a type of container).
'overt'+'urn'='OVERTURN'
(Other definitions for overturn that I've seen before include "Invert" , "Reverse (legal ruling)" , "Topple or capsize" , "Reverse (decision)" , "Reverse; upend" .)