Mother is set back by our love affair (5)
I believe the answer is:
amour
'love affair' is the definition.
(I've seen this before)
'mother is set back by our' is the wordplay.
'mother' becomes 'ma' (ma can mean mother).
'is set back' shows that the letters should be reversed in order.
'by' is a charade indicator (letters next to each other).
'ma' in reverse letter order is 'am'.
'am'+'our'='AMOUR'
(Other definitions for amour that I've seen before include "Loved one (French)" , "illicit liaison" , "A secret love affair in France" , "close relationship" , "Illicit love" .)