Firm about narcotic substance (4)
I believe the answer is:
'narcotic substance' is the definition.
The definition suggests a singular noun which matches the answer.
'firm about' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'about' becomes 'ca' (short for circa).
Can you help me to learn more?
(Other definitions for coca that I've seen before include "Drug-producing plant" , "Drug-yielding shrub" , "S. American drug crop" , "Shrub yielding drug" , "Source of drug" .)