Fed up with one blemish (6)
I believe the answer is:
deface
'blemish' is the definition.
('deface' can be a synonym of 'blemish')
'fed up with one' is the wordplay.
'fed up' becomes 'def' ('fed' going up in a down clue).
'with' means one lot of letters go next to another.
'one' becomes 'ace' ('ace' can be a synonym of 'one').
'def'+'ace'='DEFACE'
(Other definitions for deface that I've seen before include "Disfigure, spoil the surface of" , "Wantonly spoil the look of" , "six notes?" , "ruin" , "blemish" .)