Dull brown (4)
I believe the answer is:
drab
'dull brown' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for drab that I've seen before include "Dull and colourless" , "Dreary and colourless" , "Dull - dreary" , "Dull, uninteresting" , "Dowdy" .)