Butterfly perched by westbound railway (5)
I believe the answer is:
satyr
'butterfly' is the definition.
(satyr is a type of butterfly)
'perched by westbound railway' is the wordplay.
'perched' becomes 'sat' (perch can mean to take a seat).
'by' is a charade indicator (letters next to each other).
'westbound' says the letters should be written in reverse (letters written from right to left).
'railway' becomes 'ry' (abbreviation for railway).
'ry' reversed gives 'yr'.
'sat'+'yr'='SATYR'
(Other definitions for satyr that I've seen before include "Pan maybe" , "Lecherous god" , "Goatlike deity of Greek myth" , "Lecherous woodland goat-man deity" , "Pursuer of nymph" .)