American author conjuring up new nightmare, yes? (6,9)
I believe the answer is:
ernest hemingway
'american author' is the definition.
'conjuring up new nightmare yes?' is the wordplay.
'conjuring up' indicates anagramming the letters.
'new'+'nightmare'+'yes'='newnightmareyes'
'newnightmareyes' with letters rearranged gives 'ERNEST HEMINGWAY'.
(Other definitions for ernest hemingway that I've seen before include "Writer" , "A new genre is myth (anag) - author" .)