Two companies beginning to advertise drink (5)
I believe the answer is:
cocoa
'drink' is the definition.
(cocoa is a kind of drink)
'two companies beginning to advertise' is the wordplay.
'two companies' can mean two replacements for 'company'.
'company' becomes 'co' (abbreviation).
'company' becomes 'co' (abbreviation).
'beginning to' says to take the initial letters.
The initial letter of 'advertise' is 'a'.
'co'+'co'+'a'='COCOA'
(Other definitions for cocoa that I've seen before include "Light brown" , "It's made from the seed of the cacao tree" , "Lightish brown colour of a drink" , "Chocolate powder" , "Reddish brown" .)