A stony mug (5)
I believe the answer is:
stein
'mug' is the definition.
(I know that stein is a type of mug)
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 "Large earthenware beer mug" , "German earthenware beer mug" , "Inset into earthenware mug for beer" , "Inset (anag.)" , "Gertrude __, avant-garde US author, d. 1946" .)