Being green, not high on calories (6)
I believe the answer is:
callow
'being green' is the definition.
(I've seen this before)
'not high on calories' is the wordplay.
'not high' becomes 'low' ('low' is the opposite to 'high').
'on' is a charade indicator (letters next to each other).
'calories' becomes 'cal' (I can't justify this - if you can you should give a lot more credence to this answer).
'low' after 'cal' is 'CALLOW'.
Can you help me to learn more?
(Other definitions for callow that I've seen before include "Immature and inexperienced, like youth" , "Jejune" , "Green" , "Young and inexperienced" , "lacking experience of life" .)