Dull-colored (4)
I believe the answer is:
'dull-colored' 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 "Drearily dull; slattern" , "Dingy - shabby" , "Cheerless, colourless" , "Colourless like bard" , "Tedious" .)