Mark, abstract art genius (9)
I believe the answer is:
signature
'mark' is the definition.
(I have seen 'Distinctive mark ' mean 'signature' so perhaps 'mark' could also mean 'signature')
'abstract art genius' is the wordplay.
'abstract' is an anagram indicator.
'art'+'genius'='artgenius'
'artgenius' with letters rearranged gives 'SIGNATURE'.
(Other definitions for signature that I've seen before include "Identifying piece of writing, tune etc" , "Autograph" , "Squiggle perhaps" , "Indication of key" , "Distinctive mark" .)