When to act? As soon as union gets involved (2,3)
I believe the answer is:
on cue
'when to act?' is the definition.
I know nothing about this answer so I can't tell whether this works.
'as soon as union gets involved' is the wordplay.
I cannot really see how this works, but
'union' could be 'u' (common abbreviation as in 'Trades Union Congress') and 'u' is found in the answer.
'as soon as' could be 'once' (once can mean immediately after) and 'once' is found within the leftover letters.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for on cue that I've seen before include "as indicated" , "Explanation as to how something might work" , "When signal given" , "Right on time" , "End up putting chalk here before break" .)