Make colourless; whiten (6)
I believe the answer is:
bleach
I believe this is a double definition.
'make colourless' is the first definition.
(I've seen this before)
'whiten' is the second definition.
(I know that bleach is a type of whiten)
(Other definitions for bleach that I've seen before include "Strong cleaner" , "Household cleanser" , "Blanche" , "Stuff that whitens, hair or cloth" , "Peroxide" .)