At the right moment, after taking on uniform (2,3)
I believe the answer is:
on cue
'at the right moment' is the definition.
(I've seen this in another clue)
'after taking on uniform' is the wordplay.
'after' becomes 'once' (once can mean immediately after).
'taking on' means one lot of letters goes inside another.
'uniform' becomes 'U' (phonetic alphabet: alpha, bravo, charlie etc.).
'once' going around 'u' is 'ON CUE'.
(Other definitions for on cue that I've seen before include "Where a tip is appearing" , "At the correct moment, like efficient actor" , "When signal given" , "At the correct moment, like actor speaking lines" , "Explanation as to how something might work" .)