Horse of a certain color (4)
I believe the answer is:
roan
'horse of a certain color' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for roan that I've seen before include "nag" , "Horse - leather" , "Horse with brownish coat speckled with white" , "Two-colour horse" , "Horse with a brownish coat sprinkled with grey" .)