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