Deep blue dye (4)
I believe the answer is:
anil
'deep blue dye' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for anil that I've seen before include "A blue dye" , "Dye for nail" , "Indigo (plant or dye)" , "Indian man" , "It produces dye" .)