Amalgamate with a union (7)

I believe the answer is:

conjoin

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'amalgamate with a union' is the definition.
I can't judge whether this defines the answer.

'amalgamate with a union' is the wordplay.
'amalgamate with' becomes 'con' (I can't explain this - if you can you should give a lot more credence to this answer).
'a union' becomes 'join' (synonyms).
'con'+'join'='CONJOIN'

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(Other definitions for conjoin that I've seen before include "put together" , "Team up with" .)

I've seen this clue in the Evening Standard.

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