Coloring agent (3)
I believe the answer is:
dye
'coloring agent' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for dye that I've seen before include "Colour by dipping in liquid" , "Colour (fabrics)" , "Colouring agent" , "Colour or stain something" , "Colouring; sounds like fade" .)