Source of energy including nothing from chocolate substitute (5)
I believe the answer is:
carob
'chocolate substitute' is the definition.
(carob powder is a chocolate substitute)
'source of energy including nothing' is the wordplay.
'source of energy' becomes 'carb' (short for carbohydrate).
'including' is an insertion indicator.
'nothing' becomes 'O' (looks like zero - 0).
'carb' enclosing 'o' is 'CAROB'.
'from' acts as a link.
(Other definitions for carob that I've seen before include "Bean, a choc. substitute" , "(Seeds yielding) powder used as a chocolate substitute" , "Chocolate substitute pod" , "Evergreen tree with edible pods" , "Locust tree" .)