Plain to see vase is upset (8)
I believe the answer is:
overturn
'upset' is the definition.
(I've seen this before)
'plain to see vase' is the wordplay.
'plain to see' becomes 'overt' (I've seen this before).
'vase' becomes 'urn' (urn is a kind of vase).
'overt'+'urn'='OVERTURN'
'is' acts as a link.
(Other definitions for overturn that I've seen before include "Reverse (decision)" , "Capsize - invalidate" , "Invert" , "Reverse or capsize" , "Reverse (legal ruling)" .)