Is pancoast tumor rare in someone in their 30s with clean chest xray? i sometimes get discomfort in right collarbone

You don't have it: pancoast rumors virtually always have a positive CT scan; they are associated with a set of findings including pain in the shoulder, weakness and atrophy of hand muscles, and Horner's syndrome which is small pupil and eyelid drooping, appearance of smaller eye and lack of sweating, all on the side of the tumor. You need to look for another cause of your discomfort

Yes, very rare: Any type of lung cancer would be very rare in a 30 year old with a clean chest xray. Pancoast tumors are most often found in patients with long smoking history or industrial exposures. There are many things that can cause pain around the collar bone so see your doctor to try and figure out what is causing your pain