Mark of quality (9)
I believe the answer is:
character
I believe this clue is a double definition.
'mark' is the first definition.
(thesaurus)
'quality' is the second definition.
('character' can be a synonym of 'quality')
'of' is the link.
(Other definitions for character that I've seen before include "Letter or number" , "crank" , "I, for example" , "Someone in play" , "Person in a novel or play" .)