Celebrate as old lover upset group (5)
I believe the answer is:
extol
'celebrate' is the definition.
(I've seen this in another clue)
'old lover upset group' is the wordplay.
'old lover' becomes 'ex' (an ex-lover).
'upset' says the letters should be written backwards.
'group' becomes 'lot'.
'lot' back-to-front is 'tol'.
'ex'+'tol'='EXTOL'
'as' is the link.
(Other definitions for extol that I've seen before include "Praise greatly" , "Pay tribute to" , "Glorify - honour" , "Acclaim" , "Highly commend" .)