Old lady is upset by our love affair (5)
I believe the answer is:
amour
'love affair' is the definition.
(I've seen this before)
'old lady is upset by our' is the wordplay.
'old lady' becomes 'ma' (both can mean mother).
'is upset' says the letters should be written in reverse (I've seen 'being upset' mean this).
'by' means one lot of letters go next to another.
'ma' backwards is 'am'.
'am'+'our'='AMOUR'
(Other definitions for amour that I've seen before include "illicit liaison" , "A secret love affair in France" , "French love affair, usually secret" , "Romantic liaison" , "close relationship" .)