Topple plain vase (8)
I believe the answer is:
overturn
'topple' is the definition.
(I've seen this in another clue)
'plain vase' is the wordplay.
'plain' becomes 'overt' (I've seen this in other clues).
'vase' becomes 'urn' (both can mean a container).
'overt'+'urn'='OVERTURN'
(Other definitions for overturn that I've seen before include "Reverse previous findings" , "Capsize - invalidate" , "Revoke" , "Reverse or capsize" , "Reverses, a decision say" .)