Bear: one sits occasionally for artist! (5)
I believe the answer is:
'for artist' is the definition.
'bear one sits occasionally' is the wordplay.
'occasionally' means one should take alternating letters.
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" .)