This is returned, empty, by one at Oktoberfest (5)
I believe the answer is:
stein
'this is returned empty' is the definition.
The definition suggests a singular noun which matches the answer.
'by one at oktoberfest' is the wordplay.
'by' means one lot of letters go next to another.
'one at' becomes 'in' (synonymous in some cases - eg at school, in school).
'oktoberfest' becomes 'ste' (I can't explain this - if you can you should give a lot more credence to this answer).
'in' after 'ste' is 'STEIN'.
Can you help me to learn more?
(Other definitions for stein that I've seen before include "Glass" , "Inset (anag.)" , "Gertrude ---; US writer" , "Gertrude __, avant-garde US author, d. 1946" , "Earthenware beer jug" .)