Blanch when it is inside (6)
I believe the answer is:
whiten
'blanch' is the definition.
(I've seen this in another clue)
'when it is inside' is the wordplay.
'is inside' is an insertion indicator.
'when' placed around 'it' is 'WHITEN'.
(Other definitions for whiten that I've seen before include "Bleach" , "New hit (anag.)" , "free from guilt" , "Become less colourful" , "Turn pale" .)