Bottoms up! (6)
'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" .)
(Other definitions for down the hatch that I've seen before include "evokes cheers" , "(As a toast) drink up!" , "below deck? - with 9a" .)