Hillage finally opening show (5)
I believe the answer is:
event
'show' is the definition.
The answer and definition can be both acts as well as being singular nouns.
Perhaps you can see a link between them that I can't see?
'hillage finally opening' is the wordplay.
'finally' indicates one should take the final letters.
'opening' becomes 'vent' (vent is a kind of opening).
The final letter of 'hillage' is 'e'.
'e'+'vent'='EVENT'
Can you help me to learn more?
(Other definitions for event that I've seen before include "game, perhaps" , "Function" , "Fixture" , "case" , "riding competition" .)