Bleach — the WI changed name (6)
I believe the answer is:
whiten
'bleach' is the definition.
(I've seen this before)
'the wi changed name' is the wordplay.
'changed' indicates anagramming the letters.
'name' becomes 'n' (common abbreviation e.g. on forms).
'the'+'wi'='thewi'
'thewi' with letters rearranged gives 'white'.
'white'+'n'='WHITEN'
(Other definitions for whiten that I've seen before include "New hit (anag.)" , "Bleach" , "Become less colourful" , "Turn pale" , "free from guilt" .)