Firm originally cultivating a South American shrub (4)
I believe the answer is:
coca
'south american shrub' is the definition.
'firm originally cultivating a' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'originally' indicates taking the first letters.
The initial letter of 'cultivating' is 'c'.
'co'+'c'+'a'='COCA'
(Other definitions for coca that I've seen before include "Leaves for snow" , "S. American drug crop" , "Drug-yielding shrub" , "Source of drug" , "Drug-producing plant" .)