German-born surrealist (5)
I believe the answer is:
'german-born surrealist' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ernst that I've seen before include "He painted" , "Max __; Dadaist" , "Max -, twentieth century German Surrealist artist" , "Surrealist; stern (anag.)" , "artist's name" .)