Paw swelling and blister like sore

Please help identify what this is and how to treat?

Hello - you could be looking at interdigital furunculosis, arthritic joint pain exacerbated by over-long nails, trauma, a foreign body, infectious disease, a sabaceous cyst, cancer, a bite wound or something else.  The trouble is, that the presentation doesn't always reflect the cause as obviously as we would like, but I can see from the fur around it that this is sore - and being chronically licked - and that the joints don't look very comfortable, so a veterinary appointment sooner rather than later is important for this one.