A university will accept decline for so long (5)
I believe the answer is:
adieu
'so long' is the definition.
('adieu' can be a synonym of 'so long')
'a university will accept decline' is the wordplay.
'university' becomes 'u'.
'will accept' is an insertion indicator.
'decline' becomes 'die' (I've seen this in other clues).
'a'+'u'='au'
'au' enclosing 'die' is 'ADIEU'.
'for' acts as a link.
(Other definitions for adieu that I've seen before include "Farewell, ami!" , "the last word in France" , "Farewell, Monsieur!" , "See you later" , "Gallic goodbye" .)