I want to sue a real estate agent for dishonesty, failure to disclose a property defect. After an inspection radon was found.

Sounds to me like you discovered the radon exactly as an inspection might reveal.  It is unclear from your post what the real estate agent did wrong.  If the home had exposure to radon gas and the homeowner knew it but failed to disclose it on the disclosures report then you might have a reason to avoid the contract.  You do not state anything in your post that ties any sort of responsibility to the real estate agent so I am unclear what you allege he or she did wrong.  As a shorter answer, of course you can sue the real estate agent...does not mean you will win and might even get you sanctioned for brining a meritless claim.  My usual reply to the question "Can I sue...." is just that: of course you can sue - the Courts are open for business.  Pardon my sarcasm but I just want people to have realistic expectations.