Cheers in prosperous periods, entertaining times ahead (7,2)
I believe the answer is:
bottoms up
'cheers' is the definition.
(both can be said before having a drink)
'prosperous periods entertaining times ahead' is the wordplay.
'prosperous periods' becomes 'booms' (I've seen this in another clue).
'entertaining' is an insertion indicator (inviting in, as for dinner).
'times' becomes 'tt' (two abbreviations for time).
'ahead' becomes 'up' (similar in meaning).
'booms' going around 'tt' is 'bottoms'.
'bottoms'+'up'='BOTTOMS UP'
'in' acts as a link.
(Other definitions for bottoms up that I've seen before include "Down the hatch!" , "Your good health!" , "down it goes!" , "Cheers!" , "Toast" .)