Surrealist artist terribly stern (5)
I believe the answer is:
ernst
'surrealist artist' is the definition.
(painter Max Ernst)
'terribly stern' is the wordplay.
'terribly' is an anagram indicator.
'stern' is an anagram of 'ERNST'.
(Other definitions for ernst that I've seen before include "He painted" , "artist's name" , "German-born painter, Max, co-founder of dadaism, d. 1976" , "Young partner" , "Max -, twentieth century German Surrealist artist" .)