Joins cause regularly, when expected (2,3)
I believe the answer is:
on cue
'when expected' is the definition.
(I've seen this in another clue)
'joins cause regularly' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'joins'+'cause'='joinscause'
The alternate letters of 'joinscause' are 'ON CUE'.
(Other definitions for on cue that I've seen before include "At exactly the right moment" , "Right on time" , "When signal given" , "At the correct moment, like actor speaking lines" , "At the correct moment, like actor prompted to speak" .)