Eager to begin opening occasion (5)
I believe the answer is:
event
'occasion' is the definition.
(thesaurus)
'eager to begin opening' is the wordplay.
'to begin' suggests taking the first letters.
'opening' becomes 'vent' (vent is a kind of opening**).
The first letter of 'eager' is 'e'.
'e'+'vent'='EVENT'
(Other definitions for event that I've seen before include "Occasion; sports contest" , "A noteworthy happening or occurrence" , "Function" , "Item in sports programme" , "special occasion" .)