Last drink? Cheers! (7,2)
I believe the answer is:
bottoms up
'cheers' is the definition.
(both can be said before having a drink)
'last drink?' is the wordplay.
'last' becomes 'bottom' (I've seen this before).
'drink?' becomes 'sup' (to sup is to drink).
'bottom'+'sup'='BOTTOMS UP'
(Other definitions for bottoms up that I've seen before include "Toast" , "Cheers!" , "Down the hatch!" , "Your good health!" , "down it goes!" .)