Two companies with one drink (5)
I believe the answer is:
cocoa
'drink' is the definition.
(I know that cocoa is a drink)
'two companies with one' is the wordplay.
'two companies' means two substitutions for 'company'.
'company' becomes 'co' (abbreviation).
'company' becomes 'co' (abbreviation).
'with' says to put letters next to each other.
'one' becomes 'a' (a thing is one thing).
'co'+'co'+'a'='COCOA'
(Other definitions for cocoa that I've seen before include "Light brown" , "Chocolatey drink" , "Evening drink taken in rococo amphitheatre" , "product of beans!" , "Warming bedtime drink" .)