Artist's medium disputed after leaving university (7)
I believe the answer is:
matisse
'artist's' is the definition.
(Henri Matisse, 1869-1954, French artist)
'medium disputed after leaving university' is the wordplay.
'medium' becomes 'm' (as in UK clothing sizes).
'disputed' becomes 'at issue' (I've seen this in another clue).
'after leaving' indicates named letters should be taken away.
'university' becomes 'U' (abbreviation for university).
'atissue' with 'u' taken out is 'atisse'.
'm'+'atisse'='MATISSE'
(Other definitions for matisse that I've seen before include "French painter, Henri, d.1954" , "Henri -, nineteenth-twentieth century French painter/sculptor" , "Henri ---, Fauvist painter" , "Mae sits badly for the French painter" , "20th century French painter" .)