Crossword Genius

Firm about narcotic substance (4)

Ross

I believe the answer is:

coca

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'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).
'co'+'ca'='COCA'

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" .)

I've seen this clue in The Times.
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