Cleopatra's end? Cleopatra's heart stopped by this serpentine character (3)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

A clue-as-definition: A (last letter or ‘end’ of Cleopatra) P (middle letter or ‘heart’ of Cleopatra) round or ‘stopped by’ S (‘serpentine character’) – Cleopatra was killed by an asp