About to deprive of chocolate substitute (5)
I believe the answer is:
carob
'chocolate substitute' is the definition.
(I've seen this before)
'about to deprive' is the wordplay.
'about' becomes 'ca' (abbreviation for 'circa').
'to deprive' becomes 'rob' (I can't justify this - if you can you should give a lot more credence to this answer).
'ca'+'rob'='CAROB'
'of' is the link.
Can you help me to learn more?
(Other definitions for carob that I've seen before include "Locust tree" , "Tree yielding chocolate substitute" , "Arabian tree with brownish-purple pods" , "Bean, a choc. substitute" , "(Seeds yielding) powder used as a chocolate substitute" .)