Last drink delays retirement to bed (5,2)
I believe the answer is:
stays up
'delays retirement to bed' is the definition.
(to stay up is to delay going to bed)
'last drink' is the wordplay.
'last' becomes 'stay' (I've seen this before).
'drink' becomes 'sup' (to sup is to drink).
'stay'+'sup'='STAYS UP'
(Other definitions for stays up that I've seen before include "doesn't settle in the evening" , "remains air-bound!" .)