Friend is in on Saturday with the painter (7)
I believe the answer is:
matisse
'the painter' is the definition.
(Henri Matisse, 1869-1954, French artist)
'friend is in on saturday' is the wordplay.
'friend' becomes 'mate' (mate is a kind of friend).
'in' is an insertion indicator.
'on' means one lot of letters goes inside another (as in clothing 'on' a person).
'saturday' becomes 's'.
'mate' placed around 'is' is 'matise'.
'matise' placed around 's' is 'MATISSE'.
'with' is the link.
(Other definitions for matisse that I've seen before include "Henri -, nineteenth-twentieth century French painter/sculptor" , "Leading fauvist painter, d. 1954" , "Henri -, Fr. artist" , "Henri --, French artist" , "Henri ---, Twentieth Century painter" .)