Cancerous tumours are rare in clavicle
bone but any patient above age of 50 years should be treated with suspicion of cancer.
of medial end of clavicle is very dangerous as there are various neuro vascular structures around it.
A needle biopsy or a tru- cut biopsy can be undertaken after a proper radiological assessment of the lesion to determine the exact positions of various neuro vascular structures around it.
The most likely other causes for hard lump could be arthritis
of sternoclavicyular joint,fracture
of clavicle,ligamentous injury,local lipoma,carcinama etc.
You can then go for surgery after a proper biopsy.