Stain with colour (3)
I believe the answer is:
dye
'stain with colour' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for dye that I've seen before include "Liquid used to change colour of clothes" , "Colouring matter" , "Colorant" , "Colourant" , "Colour by dipping in liquid" .)