From Longman Dictionary of Contemporary Englishthink tankˈthink tank noun [countable] SSOSEa group of people with experience or knowledge of a particular subject, who work to produce ideas and give adviceright-wing/liberal/economic etc think tank a leading member of a Tory think tankGRAMMAR: Singular or plural verb?• Think tank is usually followed by a singular verb: The think tank has suggested some major reforms.• In British English, you can also use a plural verb: The think tank have suggested some major reforms.
Examples from the Corpus
think tank• That coterie would also act, as they did for the 1991 event, very much as a think tank.• The Cato Institute is a conservative think tank.• Such outright activism is not for every think tank.• This was not your familiar sighting at a Washington think tank on K Street here.• From 1989-1992, she was president of the Center for National Policy, a Washington think tank.From Longman Business Dictionarythink-tankˈthink-tank (also think tank) noun [countable] a committee of people with special experience or knowledge in a particular area which is established by a government to produce ideas and advice on somethinga government-sponsored think-tank 