Grayish brown (3)
I believe the answer is:
dun
'grayish brown' is the definition.
(synonyms)
This is all the clue.
(Other definitions for dun that I've seen before include "Ask persistently for payment of debt" , "Brownish grey horse" , "Dull greyish-brown colour" , "... to demand payment" , "Greyish brown colour" .)