Reverse open ewer (8)

I believe the answer is:

overturn

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'reverse' is the definition.
(I know that reverse can be written as overturn)

'open ewer' is the wordplay.
'open' becomes 'overt' (associated in meaning).
'ewer' becomes 'urn' (I can't justify this - if you can you should believe this answer much more).
'overt'+'urn'='OVERTURN'

Can you help me to learn more?

(Other definitions for overturn that I've seen before include "Invalidate - upset" , "Revoke" , "Capsize - invalidate" , "Reverse previous findings" , "Upset; abolish" .)

I've seen this clue in the Sydney Morning Herald.

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