Lecher perched alongside westbound track (5)
I believe the answer is:
satyr
'lecher' is the definition.
(synonyms)
'perched alongside westbound track' is the wordplay.
'perched' becomes 'sat' (perch can mean to take a seat).
'alongside' is a charade indicator (letters next to each other).
'westbound' is a reversal indicator (letters written from right to left).
'track' becomes 'ry' (abbreviation for railway).
'ry' in reverse letter order is 'yr'.
'sat'+'yr'='SATYR'
(Other definitions for satyr that I've seen before include "Woodland god; butterfly" , "Part man, part goat" , "Pursuer of nymph" , "Greek sylvan deity" , "Oversexed chap" .)