Opening for cake, when leaving Australia (4)
I believe the answer is:
gate
'opening' is the definition.
(I have seen 'Movable opening ' mean 'gate' so perhaps 'opening' could also mean 'gate')
I don't understand how the rest of the clue works.
(Other definitions for gate that I've seen before include "Hinged barrier in an entrance" , "Punish student" , "Airport boarding point" , "Facility in airport" , "Plane-boarding point" .)