An opening act or passage (7)
I believe the answer is:
doorway
'an opening' is the definition.
(I've seen this before)
'act or passage' is the wordplay.
'act' becomes 'do' (to do is to act or execute a task).
'passage' becomes 'way' (I've seen this in another clue).
'do'+'or'+'way'='DOORWAY'
(Other definitions for doorway that I've seen before include "Room access, threshold" , "Entrance to a room" , "Opening" , "Threshold, portal" .)