Hard, painless lump on left clavicle, approx 2 cm or so. could it be cancer?

Advise: you see your doctor for exam. It is not possible to give you guidance without a dedicated history, physical exam, and, possibly imaging studies.

Suspicious lump: Any new suspicious lump that does not go away after a week should be evaluated by a physician . Absence of pain does not guarantee that it is benign.

Not likely, but: A cancer of the clavicle bone would be highly unlikely. However, there are lymph nodes located just behind the mid point of the clavicle that might be enlarged and mistaken for the bone itself. Enlargement here would be significant. If this painless lump has been present for years without change likely insignificant. If it's just been noticed more recently, or getting larger, have it examined .

Yes: Supraclavicular nodes on left side in close proximity to clavicle can represent mets to Virchows nodes. These are nodes draining the left pelvis into mediastinal lymphatics and this group of nodes. They can also represent spread from any palpable nodes in left axilla. If not nodal then think of lesion of clavicle such as chondroma.