Visually dull (4)
I believe the answer is:
drab
'visually dull' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for drab that I've seen before include "Colourless like bard" , "Dreary and colourless" , "Grey or light brown cloth" , "Gloomy" , "Not cheering" .)