Food for a dog going by railway (5)
I believe the answer is:
curry
'food' is the definition.
(curry is a kind of food)
'a dog going by railway' is the wordplay.
'a dog' becomes 'cur' (cur is a kind of dog).
'going by' is a charade indicator (letters next to each other).
'railway' becomes 'ry' (abbreviation for railway).
'cur'+'ry'='CURRY'
'for' acts as a link.
(Other definitions for curry that I've seen before include "Highly spiced dish" , "Korma, for example" , "Hot spicy dish" , "Vindaloo, for example" , "Pungent Indian dish" .)