ˌclosed ˈshop noun [countable]
a company, factory etc where all the workers must belong to a particular trade union

[TahlilGaran] Dictionary of Contemporary English ▲

closed shop
n. phr.
1. A plant or factory that employs only union workers.
Our firm has been fighting the closed shop policy for many years now.
2. A profession or line of work dominated by followers of a certain mode of thinking and behaving that does not tolerate differing views or ideas.
Certain groups of psychologists, historians, and linguists often behave with a closed-shop mentality.
Antonym: OPEN SHOP.

[TahlilGaran] English Idioms Dictionary ▲