Too much one would set back as before (5)
I believe the answer is:
ditto
'as before' is the definition.
(I've seen this before)
'too much one would set back' is the wordplay.
'too much' becomes 'ott' (short for 'over the top' - informal British).
'one' becomes 'i' (Roman numeral).
'would' becomes 'd' (I've seen this in other clues).
'set back' says the letters should be written backwards.
'ott'+'i'+'d'='ottid'
'ottid' written backwards gives 'DITTO'.
(Other definitions for ditto that I've seen before include "Repeat (instruction)" , "Dot it for the same again" , "Duplicate" , "Notation meaning the same again" , "Two dots indicating the same again" .)