At the right moment (2,3)
I believe the answer is:
on cue
'at the right moment' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for on cue that I've seen before include "you can find chalk here" , "When expected" , "As a snooker player might be" , "Pat" , "At the correct moment, like actor prompted to speak" .)