Horse with brownish coat speckled with white or grey (4)
I believe the answer is:
roan
I believe this clue is a double definition.
'horse with brownish coat speckled with white' is the first definition.
(I've seen this before)
'grey' is the second definition.
(thesaurus)
(Other definitions for roan that I've seen before include "Reddish-coloured horse" , "Red-coloured horse" , "Horse with a brownish coat sprinkled with grey" , "Horse with main colour interspersed with hairs of another" , "Of horse, with coat sprinkled with white or grey hairs" .)