Whiten (6)

I believe the answer is:
bleach
'whiten' is the definition.
(I know that bleach is a type of whiten)
This is all the clue.
(Other definitions for bleach that I've seen before include "Agent that makes things white or colourless" , "Strong cleaner" , "Whiten - disinfectant" , "Purge" , "Make white by sun or chemicals" .)