A Claymore is a human who gets their powers by having pieces of the yoma monsters put inside their body by the organization, to turn them into super beings capable of defeating yoma.
When a Claymore 'Awakens' they basically let the monster in them take over and they cannot go back, that's why the other claymores are sent after them to destroy them. They can pretend to be humanoid but they'll still have the insatiable thirst for human guts, just like the yoma monsters.
When Priscilla killed Teresa, Clare asked the Organization to turn her into a Claymore but to use Teresa's remains instead of a Yoma. Clare's true awakening was not that of an awakened being, but of Teresa.
Once she 'awakened' Teresa, Teresa killed Priscilla but priscilla's hatred and body remained. Priscilla's pure hatred was too much for her so to battle her, she awakened her yoma powers in this issue
