Trouble coming up - single lad has an affair (7)
I believe the answer is:
liaison
'an affair' is the definition.
('liaison' can be a synonym of 'affair')
'trouble coming up single lad' is the wordplay.
'trouble' becomes 'ail' ('ail' can be a synonym of 'trouble').
'coming up' shows that the letters should be reversed in order.
'single' becomes 'i' (one).
'lad' becomes 'son'.
'ail' in reverse letter order is 'lia'.
'lia'+'i'+'son'='LIAISON'
'has' acts as a link.
(Other definitions for liaison that I've seen before include "An affair or mediation" , "Mediator" , "Collaboration" , "A contact or affair" , "Fling - intrigue" .)