Union act is a reversal (1,4)
I believe the answer is:
u turn
'reversal' is the definition.
(u-turn is a kind of reversal)
'union act' is the wordplay.
'union' becomes 'U' (common abbreviation as in 'Trades Union Congress').
'act' becomes 'turn' (theatre).
'u'+'turn'='U-TURN'
'is a' acts as a link.
I've seen this clue in The Daily Express.
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