I suffer over boy's affair (7)
I believe the answer is:
liaison
'affair' is the definition.
('liaison' can be a synonym of 'affair')
'i suffer over boy's' is the wordplay.
'suffer' becomes 'ail' (ailing is a kind of suffering).
'over' says the letters should be written in reverse.
'boy' becomes 'son' ('son' can be a synonym of 'boy'**).
'i'+'ail'='iail'
'iail' reversed gives 'liai'.
'liai'+'son'='LIAISON'
(Other definitions for liaison that I've seen before include "Intermediary" , "entanglement" , "Naughty affair?" , "carryon" , "Connection - relationship" .)