Oberon has right to replace Puck at last but gets fungal growth (9)

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

FAIRY [k] ING (Oberon, king of the fairies, for example in Shakespeare’s A Midsummer Night’s Dream ), with R (abbreviation for right) replacing the K which is the last letter of [puc]K. A circular ring of mushrooms in an area of grass, or a ring of darker or vigorously-growing grass caused by fungi in the soil underneath.