Upset an open container (8)
I believe the answer is:
overturn
'upset' is the definition.
(I know that upset can be written as overturn)
'an open container' is the wordplay.
'an open' becomes 'overt' (I've seen this before).
'container' becomes 'urn' (urn is a kind of container).
'overt'+'urn'='OVERTURN'
(Other definitions for overturn that I've seen before include "Reverse; upend" , "Revoke" , "Topple or capsize" , "Crash" , "Capsize - invalidate" .)