A sitter becoming performer (7)
I believe the answer is:
artiste
'performer' is the definition.
(artiste is a kind of performer)
'a sitter becoming' is the wordplay.
'becoming' indicates anagramming the letters.
'sitter' with letters rearranged gives 'rtiste'.
'a'+'rtiste'='ARTISTE'
(Other definitions for artiste that I've seen before include "Professional performer" , "Tears it (anag.)" , "Attires for professional entertainer" , "Performers" , "Attires (anag.)" .)