Reddish horse (4)
I believe the answer is:
roan
'reddish horse' 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 "Red-coloured horse" , "Type of 17's coat colouring" , "nag" , "Horse with coat speckled with white or grey" , "Bay spotted" .)