Bear: one sits occasionally for artist! (5)
I believe the answer is:
ernst
'for artist' is the definition.
(max ernst)
'bear one sits occasionally' is the wordplay.
'occasionally' means one should take alternating letters.
'bear'+'one'+'sits'='bearonesits'
The alternate letters of 'bearonesits' are 'ERNST'.
(Other definitions for ernst that I've seen before include "Young partner" , "Max --, German painter" , "Max __; Dadaist" , "German-born painter, Max, co-founder of dadaism, d. 1976" , "artist's name" .)