Sing the praises of former lover set to come back (5)
I believe the answer is:
extol
'sing the praises of' is the definition.
(I've seen this before)
'former lover set to come back' is the wordplay.
'former lover' becomes 'ex'.
'set' becomes 'lot' ('lot' can be a synonym of 'set').
'to come back' says the letters should be written backwards.
'lot' backwards is 'tol'.
'ex'+'tol'='EXTOL'
(Other definitions for extol that I've seen before include "Pay tribute to" , "Say great things about" , "Praise with enthusiasm" , "'Praise, glorify (5)'" , "Magnify" .)