Catch-22 is a situation where conflicting rules make the desired outcome impossible. It comes from a novel by the American author Joseph Heller, in which pilots would not have to fly missions if they were mentally ill, but not wanting to fly dangerous missions was held to be proof of sanity, so they had to fly anyway.
('Catch 22', without the hyphen, is also used.)
(USA) If you circle the wagons, you stop communicating with people who don't think the same way as you to avoid their ideas. It can also mean to bring everyone together to defend a group against an attack.
If something is as clean as a whistle, it is extremely clean, spotless. It can also be used to mean 'completely', though this meaning is less common nowadays. If somebody is clean as a whistle, they are not involved in anything illegal.