Personage finally opening show (5)
I believe the answer is:
event
'show' is the definition.
(event can mean a show or exhibition)
'personage finally opening' is the wordplay.
'finally' indicates one should take the final letters.
'opening' becomes 'vent' (a vent is an opening or outlet).
The final letter of 'personage' is 'e'.
'e'+'vent'='EVENT'
(Other definitions for event that I've seen before include "'Occurrence, happening (5)'" , "Incident - final outcome" , "pole vault maybe" , "Notable occasion" , "celebration?" .)