Confederacy or those who opposed it? (5)

I believe the answer is:

union

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'confederacy' is the definition.
(thesaurus)

I don't understand the rest of the clue.

(Other definitions for union that I've seen before include "Organisation of employees for bargaining purposes" , "See 22" , "Result of joining" , "America" , "Group who may want better conditions" .)

I've seen this clue in The Times and The Sun.

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