Mother is upset over our love affair (5)
I believe the answer is:
amour
'love affair' is the definition.
(I've seen this before)
'mother is upset over our' is the wordplay.
'mother' becomes 'ma' (ma can mean mother).
'is upset over' says the letters should be written backwards (I've seen 'are upset over' mean this).
'ma' in reverse letter order is 'am'.
'am'+'our'='AMOUR'
(Other definitions for amour that I've seen before include "20 [AFFAIR]" , "Secretive affair" , "Fling" , "Affair of the heart" , "Affaire" .)