Companies originally advertising chocolate drink (5)
I believe the answer is:
cocoa
'chocolate drink' is the definition.
(I've seen this in another clue)
'companies originally advertising' is the wordplay.
'companies' becomes 'coco' (co for company, twice).
'originally' says to take the initial letters.
The initial letter of 'advertising' is 'a'.
'coco'+'a'='COCOA'
(Other definitions for cocoa that I've seen before include "Kind of chocolate drink" , "Bean used in making chocolate" , "Evening drink taken in rococo amphitheatre" , "Chocolate-like drink" , "Hot brown drink" .)