Last drink? Cheers! (7,2)
I believe the answer is:
'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).
(Other definitions for bottoms up that I've seen before include "Toast" , "Cheers!" , "Down the hatch!" , "Your good health!" , "down it goes!" .)