A less than amorous advance in the subway! (9)
I believe the answer is:
underpass
'subway' is the definition.
('underpass' can be a synonym of 'subway')
'a less than amorous advance' is the wordplay.
'a less than' becomes 'under' (I've seen this before).
'amorous advance' becomes 'pass' (I've seen this before).
'under'+'pass'='UNDERPASS'
'in the' is the link.
(Other definitions for underpass that I've seen before include "way across road?" , "way below the level of others making progress" , "Tunnel beneath a road" .)