Subdermal lymph node or lipoma
A single, pea size lump at the base of the neck, just under the skin are descriptive of a subdermal lymph node or lipoma. They have no significance or cannot be related to any disease process exactly.
I suggest you to a 'Wait and Watch' policy. If it grows in size, you need a better evaluation by a doctor.
A clot is just firm and will reduce in size as they days go on.
Hope I allayed your anxiety.