Slate-colored (4)
I believe the answer is:
gray
'slate-colored' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for gray that I've seen before include "In America, dull" , "Thomas --, poet" , "English poet" , "a bit gloomy for the Americans" , "An Anatomist; an Elegist" .)