Heap praise on former lover, upsetting group (5)
I believe the answer is:
extol
'heap praise on' is the definition.
(as in extolling someone's virtues)
'former lover upsetting group' is the wordplay.
'former lover' becomes 'ex' (an ex-lover).
'upsetting' shows that the letters should be reversed in order.
'group' becomes 'lot'.
'lot' reversed gives 'tol'.
'ex'+'tol'='EXTOL'
(Other definitions for extol that I've seen before include "Lavishly praise" , "Praise greatly" , "be lavish with praise" , "Praise highly, laud" , "Exalt" .)