closed shop 
noun
1A place of work where all employees must belong to an agreed trade union. Compare with union shop.Undoubtedly the freedom of association provisions were drafted with the intent of attacking unions, to break down so-called closed shops....
- Some business sectors in Asia were once regarded by the West as a closed shop, but deregulation is now in full stride as corporate and political leaders strive to restructure and increase competitiveness.
- The Publish-Or-Perish syndrome has always been a fact of life for academics, and in earlier times was the subject of much amusement both within and without the fairly closed shop of the scientific world.
1.1 [in singular] The system in operation in a place of work that is a closed shop: the outlawing of the closed shop...
- While the trade unions still control access to employment in the sector through a closed-shop policy, they are in no position to present any serious challenge.
- The interested amateur is likely to be repelled and hurt by this attitude of closed-shop trade-unionism.
- By contrast, union members - especially in closed-shop industries - cannot simply renounce their union memberships, unless they're also willing to quit their jobs.
1.2An area of activity that is restricted to a particular small group: it’s a very closed shop, the pharmaceutical world...
- In the case of the Bahamas, the government is a closed shop.
- Despite all its talk of diversity, big media in American tends to be something of a closed shop.
- The Premiership's top three is something of a closed shop.