It is a chop going off, becoming light green (9)
I believe the answer is:
pistachio
'light green' is the definition.
(I've seen this before)
'it is a chop going off' is the wordplay.
'going off' indicates anagramming the letters.
'it'+'is'+'a'+'chop'='itisachop'
'itisachop' with letters rearranged gives 'PISTACHIO'.
'becoming' is the link.
(Other definitions for pistachio that I've seen before include "Type of nut" , "Edible nut" , "nuts" , "Small sweet-tasting nut" , "Nut - it is a chop (anag)" .)