Two companies with one drink (5)
I believe the answer is:
cocoa
'drink' is the definition.
(cocoa is a kind of drink)
'two companies with one' is the wordplay.
'two companies' means two substitutions for 'company'.
'company' becomes 'co' (abbreviation).
'company' becomes 'co' (abbreviation).
'with' means one lot of letters go next to another.
'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" .)