One enlarged tonsil, could it be a cancer?

Not likely.: Usually, if a tonsil is enlarged, it is associated with infection. Acute pharyngitis has many causes, gp. A beta-hemolytic streptococcus, n. Gonorrhoe, and c. Diphtheriae being notable. Patients have tonsillar asymmetry, and a "hot potato voice". Oral cancer patients have non-healing ulcers that can be painful. Cervical and/or supraclavicular lymph nodes can be rock hard and enlarged.

I doubt, but...: While it is true that there are a lot of things that can make the tonsils enlarged- and most commonly is infections - cancer in the tonsil can happen and is also presented with enlargement of the tonsil. If you are in doubt- go to see your doctor and perhaps an ENT if that looks suspicious. They will evaluate you and perhaps removing the tonsil if that looks suspicious.

Could be: How long has the unilateral enlargement lasted? What are associated symptoms? Is there lymph node enlargement? It is not feasible to give a reasonable answer without examining the lesion and may be taking a biopsy for examination of the tissue by a pathologist. It would be prudent to consult your doctor.