The same one not drinking during party (5)
I believe the answer is:
ditto
'the same' is the definition.
(ditto means the same as previously mentioned)
'one not drinking during party' is the wordplay.
'one not drinking' becomes 'tt' (short for teetotal).
'during' is an insertion indicator.
'party' becomes 'dio' (I can't justify this - if you can you should believe this answer much more).
'tt' put within 'dio' is 'DITTO'.
Can you help me to learn more?
(Other definitions for ditto that I've seen before include "The same thing" , "Repeat the word above" , "Duplicate" , "As given above" , "Two dots indicating the same again" .)