Bleach when it is dissolved (6)
I believe the answer is:
whiten
'bleach' is the definition.
(I've seen this in another clue)
'when it is dissolved' is the wordplay.
'is dissolved' indicates an anagram.
'when'+'it'='whenit'
'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" .)