Star of toilet graffiti? (6)
I believe the answer is:
bogart
'star' is the definition.
(Humphrey Bogart 1889-1957)
'toilet graffiti?' is the wordplay.
'toilet' becomes 'bog' (informal term for toilet).
'graffiti?' becomes 'art' (graffiti is a type of art).
'bog'+'art'='BOGART'
'of' acts as a link.
(Other definitions for bogart that I've seen before include "Lauren Bacall's first actor husband, d. 1957" , "See 12" , "Rob gat from the old film star" , "American actor" , "Famous player" .)