Player makes a mess of a sitter (7)
I believe the answer is:
artiste
'player' is the definition.
(I've seen this before)
'a mess of a sitter' is the wordplay.
'mess of' indicates an anagram (I've seen 'mess' mean this).
'sitter' with letters rearranged gives 'rtiste'.
'a'+'rtiste'='ARTISTE'
'makes' is the link.
(Other definitions for artiste that I've seen before include "Theatre performer" , "Entertainer, singer or dancer" , "A sitter for creative person" , "Attires for professional entertainer" , "Player" .)