Union with not a single member! (7)

I believe the answer is:

wedlock

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'union' is the definition.
(synonyms)

I cannot understand how the remainder of the clue works.

(Other definitions for wedlock that I've seen before include "The married state - throw away the key" , "A state of union" , "Matrimony" , "Marital state" , "State of marriage" .)

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