Rising in boozer, son saying “cheers” (7,2)
I believe the answer is:
bottoms up
'cheers' is the definition.
(both can be said before having a drink)
'rising in boozer son saying' is the wordplay.
'rising' says the letters should be written in reverse (going up in a down clue).
'in' means one lot of letters goes inside another.
'boozer' becomes 'pub' (UK slang).
'son' becomes 's' (genealogical abbreviation for son).
'saying' becomes 'motto' (motto is a kind of saying).
's'+'motto'='smotto'
'pub' placed around 'smotto' is 'pusmottob'.
'pusmottob' reversed gives 'BOTTOMS UP'.
(Other definitions for bottoms up that I've seen before include "Toast" , "down it goes!" , "Down the hatch!" , "Your good health!" , "Cheers!" .)