Fairy queen overwhelms Athenian character's heart (4)

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

ER (the late Queen Elizabeth II), replacing the middle letter (heart) of either P [h] I or P [s] I (Greek letters = Athenian characters). Old word for a fairy (as in the G&S comic opera Iolanthe, or The Peer and the Peri ). Perhaps an extended definition based on Shakespeare’s A Midsummer Night’s Dream , which features the King and Queen of the Fairies plus a company of assorted Athenians and some messing about with a love potion.