Celebrate former partner's reversal of fortune (5)
I believe the answer is:
extol
'celebrate' is the definition.
(I've seen this before)
'former partner's reversal of fortune' is the wordplay.
'former partner' becomes 'ex'.
'reversal of' says the letters should be written backwards.
'fortune' becomes 'lot' (as in a person's lot in life).
'lot' backwards is 'tol'.
'ex'+'tol'='EXTOL'
(Other definitions for extol that I've seen before include "Say great things about" , "Warmly approve of" , "Praise highly, laud" , "Sing the praises of" , "Pay tribute to" .)