Supplement causing dependence about to be withdrawn (8)
I believe the answer is:
additive
'supplement' is the definition.
(I've seen this before)
'causing dependence about to be withdrawn' is the wordplay.
'causing dependence' becomes 'addictive'.
'about' becomes 'c' (abbreviation for 'circa').
'to be withdrawn' is a deletion indicator.
'addictive' with 'c' taken out is 'ADDITIVE'.
(Other definitions for additive that I've seen before include "Substance stirred in to colour or flavour food" , "Colouring maybe" , "Such as a colorant or preservative in foodstuffs" , "Something joined, to food say" , "Eg, a colorant or preservative in foods" .)