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.

Want a hint initially instead of a full solution? Install my app