Drinking toast (7,2)
(Other definitions for bottoms up that I've seen before include "Your good health!" , "down it goes!" , "Toast" , "Cheers!" , "Down the hatch!" .)
'drinking toast' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for cheers that I've seen before include "Shouts of praise" , "Shouts of approval" , "Something said before drinking" , "Thank you; goodbye!" , "Hurrahs - it's a toast" .)