Cover with graffiti (6)
I believe the answer is:
deface
'cover with graffiti' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for deface that I've seen before include "Spoil surface of, mar" , "Deliberately spoil the appearance of something" , "Wantonly spoil the look of" , "Spoil or damage the surface of" , "Mar or spoil the appearance of" .)