Wretch going by railway to search for a grooming aid (9)
I believe the answer is:
currycomb
'a grooming aid' is the definition.
(a comb for horses)
'wretch going by railway to search' is the wordplay.
'wretch' becomes 'cur' (I've seen this before).
'going by' is a charade indicator (letters next to each other).
'railway' becomes 'ry' (abbreviation for railway).
'to search' becomes 'comb' (combing is a kind of searching).
'cur'+'ry'+'comb'='CURRYCOMB'
'for' is the link.
(Other definitions for currycomb that I've seen before include "Horse is treated with this" , "Horse-grooming tool" .)