Party involves one not drinking as previously (5)
I believe the answer is:
'party involves one not drinking as previously' is the definition.
I can't judge whether this defines the answer.
'party involves one not drinking as previously' is the wordplay.
'party' becomes 'do' (do is a kind of party).
'involves' is an insertion indicator.
'one not drinking' becomes 'tt' (short for teetotal).
'as' becomes 'i' (I can't explain this - if you can you should believe this answer much more).
'previously' says to put letters next to each other.
'tt' after 'i' is 'itt'.
'do' enclosing 'itt' is 'DITTO'.
Can you help me to learn more?
(Other definitions for ditto that I've seen before include "Two small marks indicating the same again" , "Copy" , "Notation meaning the same again" , "Repeat the word(s) above" , "Repeat (instruction)" .)