Bleach when it is dissolved (6)
I believe the answer is:
'bleach' is the definition.
(I've seen this in another clue)
'when it is dissolved' is the wordplay.
'is dissolved' indicates an anagram.
'whenit' anagrammed gives 'WHITEN'.
(Other definitions for whiten that I've seen before include "free from guilt" , "Bleach" , "New hit (anag.)" , "Become less colourful" , "Turn pale" .)