Question

This requires employees who are members of the union before being hired to remain as members once they are hired, unless they take advantage of an "escape clause."

Answer 1

Maintenance of membership agreement requires employees who are members of the union before being hired to remain as members once they are hired, unless they take advantage of an 'escape clause'.
Escape clause refers to a set of conditions within a contract that allows a party to completely avoid any requirements in the contract as long as that party fulfills a previously agreed set of conditions.

Answer 2

Maintenance of membership agreement requires employees who are members of the union before being hired to remain as members once they are  hired, unless they take advantage of an 'escape clause'.