The last of university dons live so long (3)
I believe the answer is:
bye
'so long' is the definition.
('bye' can be a synonym of 'so long')
'the last of university dons live' is the wordplay.
'the last of' suggests the final letters.
'dons' indicates putting letters inside (don means to wear or put on).
'live' becomes 'be' ('be' can be a synonym of 'live').
The final letter of 'university' is 'y'.
'y' placed within 'be' is 'BYE'.
(Other definitions for bye that I've seen before include "Run not off bat" , "Cricketing extra" , "See you" , "Get one to advance to the next round without playing an opponent" , "Run scored in cricket from unhit ball" .)