Deep blue pigment (11)
I believe the answer is:
ultramarine
'deep blue pigment' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for ultramarine that I've seen before include "See a colour" , "Vivid blue" , "Blue pigment originally made from lapis lazuli" , "Deep blue pigment" , "Brilliant deep blue" .)