Artist's friend cuddling missus regularly (7)
I believe the answer is:
matisse
'artist's' is the definition.
(Henri Matisse, 1869-1954, French artist)
'friend cuddling missus regularly' is the wordplay.
'friend' becomes 'mate' (mate is a kind of friend).
'cuddling' indicates putting letters inside (inserted letters are cuddled or held).
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
The alternating letters of 'missus' are 'iss'.
'mate' enclosing 'iss' is 'MATISSE'.
(Other definitions for matisse that I've seen before include "Henri ---, Fauvist painter" , "Henri --, French artist" , "French impressionist" , "Henri -, Fr. artist" , "Henri -, C twenty Fr. painter, sculptor, illustrator" .)