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' means one lot of letters go next to another.
'westbound' says the letters should be written in reverse (letters written from right to left).
'track' becomes 'ry' (abbreviation for railway).
'ry' back-to-front 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" .)