Academic opener for an ass (6)
I believe the answer is:
donkey
'for an ass' is the definition.
(donkey is a kind of ass)
'academic opener' is the wordplay.
'academic' becomes 'don' (type of academic - especially at Oxbridge).
'opener' becomes 'key' (keys open things).
'don'+'key'='DONKEY'
(Other definitions for donkey that I've seen before include "Domesticated animal" , "Burro" , "Nativity play animal?" , "stubborn type" , "Garment associated with building workers" .)