Bottoms up! (6)
I believe the answer is:
cheers
'bottoms up' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for cheers that I've seen before include "Heartens with hurrahs" , "Praises < eggs on" , "Encouragement" , "Ta - ta-ta" , "Hurrahs and oles" .)