From clear.com: How secure is it?
Your CLEAR connection is very secure. Unlike WiFi, CLEAR technology uses a licensed 2.5 Ghz frequency and OFDM transmission protocol for a very secure connection.
The combination of licensed frequencies and OFDM technology provides a very secure connection.
This is a very strange statement, since neither the frequency nor how it's multiplexed have anything whatsoever to do with security. Their customer support says the link is encrypted, but the web site makes no mention of this.