Colouring substance to fade, we hear (3)
I believe the answer is:
dye
'colouring substance' is the definition.
(I've seen this before)
'fade we hear' is the wordplay.
'fade' becomes 'die' (fade can mean to die away).
'we hear' shows a homophone (sound like).
'die' sounds like 'DYE'.
'to' is the link.
(Other definitions for dye that I've seen before include "Colorant" , "Soluble substance for staining or colouring" , "Tint" , "Colour (fabrics)" , "Colour or stain something" .)