I suffer over lad's affair (7)
I believe the answer is:
liaison
'affair' is the definition.
('liaison' can be a synonym of 'affair')
'i suffer over lad's' is the wordplay.
'suffer' becomes 'ail' (ailing is a kind of suffering).
'over' says the letters should be written in reverse.
'lad' becomes 'son'.
'i'+'ail'='iail'
'iail' back-to-front is 'liai'.
'liai'+'son'='LIAISON'
(Other definitions for liaison that I've seen before include "Secret love-affair" , "Binding agent of a sauce; secret relationship" , "Mediator" , "sexual adventure" , "Contact between groups" .)