Possible Clues:

• Wall scribbles
• Some wall art
• Spray-painted writing
• Scribblings on the walls of buildings
• Good artist, fellow appropriate to lead one drawing on wall
• More than one drawing on the wall
• What 20-, 35-, and 55-Across have all been hit with
• Unauthorised public writing
• Maybe the writing's on the wall
• Scrawls on walls - fair gift