Being a mug, is stony (5)
I believe the answer is:
stein
'being' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for stein that I've seen before include "Gertrude __, avant-garde US author, d. 1946" , "German earthenware beer mug" , "Inset into earthenware mug for beer" , "Large earthenware beer mug" , "Inset (anag.)" .)