A German mug (5)
I believe the answer is:
stein
'mug' is the definition.
(stein is a kind of mug)
'a german' is the wordplay.
I cannot quite see how this works, but
'a german' could be 'ein' ('a' in German) and 'ein' is located in the answer.
The remaining letters 'st' is a valid word which might be clued in a way I don't understand.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for stein that I've seen before include "German earthenware beer mug" , "Inset into earthenware mug for beer" , "Gertrude __, avant-garde US author, d. 1946" , "Large earthenware beer mug" , "Inset (anag.)" .)