Concern had doubled over a drink (5)
I believe the answer is:
cocoa
'drink' is the definition.
(I know that cocoa is a drink)
'concern had doubled over a' is the wordplay.
'concern' becomes 'co' (short for company - concern can mean a business).
'had' means one lot of letters go next to another.
'doubled over' becomes 'co' (I am not sure about this - if you are sure you should believe this answer much more).
'co'+'co'+'a'='COCOA'
Can you help me to learn more?
(Other definitions for cocoa that I've seen before include "Evening drink taken in rococo amphitheatre" , "Main constituent of chocolate" , "Light brown" , "Hot bedtime drink" , "Powdered cacao seeds" .)