As before, single abstainer in party (5)
I believe the answer is:
ditto
'as before' is the definition.
(I've seen this before)
'single abstainer in party' is the wordplay.
'single' becomes 'i' (one).
'abstainer' becomes 'tt' (I've seen this in other clues).
'in' indicates putting letters inside.
'party' becomes 'do' (do is a kind of party).
'i'+'tt'='itt'
'itt' inserted inside 'do' is 'DITTO'.
(Other definitions for ditto that I've seen before include "Same again, indicated by two dots" , "Two dots indicating the same again" , "Two small marks indicating the same again" , "Repeat the word above" , "Mark used to indicate the word above it should be repeated" .)