Trouble coming up - single lad has an affair (7)
I believe the answer is:
'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'.
'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" .)