Two firms beginning to advertise drink (5)
I believe the answer is:
cocoa
'drink' is the definition.
(I know that cocoa is a drink)
'two firms beginning to advertise' is the wordplay.
'two firms' can mean two replacements for 'firm'.
'firm' becomes 'co' (abbreviation for company).
'firm' becomes 'co' (abbreviation for company).
'beginning to' says to take the initial letters.
The first letter of 'advertise' is 'a'.
'co'+'co'+'a'='COCOA'
(Other definitions for cocoa that I've seen before include "Hot chocolate drink" , "Popular chocolate drink" , "Chocolate-like drink" , "Hot beverage" , "Chocolate ingredient" .)