One-time rising group crack up? (5)
I believe the answer is:
extol
'crack up?' is the definition.
(cracking up is a kind of extolling)
'one-time rising group' is the wordplay.
'one-time' becomes 'ex'.
'rising' says the letters should be written backwards (going up in a down clue).
'group' becomes 'lot' (lot is a kind of group**).
'lot' in reverse letter order is 'tol'.
'ex'+'tol'='EXTOL'
(Other definitions for extol that I've seen before include "Crack up" , "Talk up" , "Speak highly" , "Speak well of" , "Lift up" .)