Free union (4,8)

I believe the answer is:

open marriage

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'free union' is the definition.
The definition suggests a singular noun which matches the answer.

'free union' is the wordplay.
'free' becomes 'open' (similar in meaning).
'union' becomes 'marriage' (synonyms).
'open'+'marriage'='OPEN MARRIAGE'

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(Other definitions for open marriage that I've seen before include "Union in which partners have great freedom" , "Union in which the partners do not require fidelity" .)

I've seen this clue in The Guardian.

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