The catch is the free CLEAR membership is given to you when you become a diamond.
CLEAR is great. Typically, there is no line and no waiting. However, you still merge with the regular TSA Pre line for the luggage and security inspection in the end. The $179 annual fee is waived and you can add a family member for $50 a year.
This is my second year having CLEAR as a diamond. Totally loving it.