Wretch going by railway to search for a grooming aid (9)
I believe the answer is:
Here is my best explanation:
'a grooming aid' is the definition.
(a comb for horses)
'wretch going by railway to search' is the subsidiary indication.
'wretch' becomes 'cur' (**).
'going by' means one lot of letters go next to another.
'railway' becomes 'ry'.
'to search' becomes 'comb' (combing is a kind of searching).
'for' is the link.
(Other definitions for currycomb that I've seen before include "Horse is treated with this" and "Horse-grooming tool".)