A little diplomat is selecting portraitist (7)
I believe the answer is:
matisse
'portraitist' is the definition.
(Henri Matisse, 1869-1954, the French artist)
'a little diplomat is selecting' is the wordplay.
'a little' indicates the answer is hidden within the clue.
'MATISSE' can be found hidden inside 'diplomat is selecting'.
(Other definitions for matisse that I've seen before include "20th century French painter" , "Henri ---, Fauvist painter" , "Henri -, Fr. artist" , "French impressionist" , "Henri -, nineteenth-twentieth century French painter/sculptor" .)