Better, except when editor’s replacing short part (6)
I believe the answer is:
exceed
'better' is the definition.
(I've seen this before)
'except when editor's replacing short part' is the wordplay.
I cannot quite see how this works, but
'when' could be 'e' (I've seen this in other clues) and 'e' is present in the answer.
'editor' could be 'ed' and 'ed' is found in the answer.
This explanation may well be incorrect...
(Other definitions for exceed that I've seen before include "Go beyond, expectations perhaps" , "Outstrip" , "Bud used as spice" , "Go beyond the limit or norm" , "Outperform" .)