If margin is clear could cancer cells still be left behind after surgery?

Yes: The goal of every cancer surgery is to remove the cancer and a rim of normal tissue to "clear margins". Despite this, there may be microscopic cancer left behind, either in the surgical area or elsewhere in the body. This is why we often recommend radiation- and chemotherapy --to treat this local or systemic disease, respectively. Regrettably, it's what we don't see that matters most.

Insightful question!: Some cancers shed, send their cells or even grow simutaneously far away. At an early phase and stage, tumor cells are detected in the blood and bone marrow. Clear margins are somewhat assuring that local recurrence in the area will not come back. In areas nearby, other foci might be hiding to grow later and cause local recurrence or even metastasize some place far away. Be specific, what cancer?