Blue plant dye (6)
I believe the answer is:
indigo
'blue plant dye' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for indigo that I've seen before include "Colour - I'd go in (anag)" , "Tropical plant" , "supplier of blue stuff" , "Deep-blue dye or colour" , "A deep blue dye or colour" .)