Dark blue dye (4)
'dark blue dye' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for indigo that I've seen before include "type of dye" , "Blue-violet colour" , "Colour - I'd go in (anag)" , "Sixth rainbow colour" , "Deep-blue dye or colour" .)
(Other definitions for anil that I've seen before include "Dye for nail" , "Indigo (plant or dye)" , "name of man" , "Species of blue" , "Source of dye" .)