Happening to be making one's entrance on stage (5,2)
I believe the answer is:
going on
'happening' is the definition.
('go on' can be a synonym of 'happen')
'be making one's entrance on stage' is the wordplay.
I cannot quite see how this works, but
'be' could be 'go' (going is a kind of being) and 'go' is located in the answer.
'on' is within the answer.
'one' could be 'i' (Roman numeral) and 'i' is found within the answer.
This explanation may well be incorrect...
'to' acts as a link.
Can you help me to learn more?
(Other definitions for going on that I've seen before include "Continuing" , "Almost" , "reluctant to stop?" .)