Monster rewrites evil article about hat design (9)
I believe the answer is:
leviathan
'monster' is the definition.
(I know that leviathan is a type of monster)
'rewrites evil article about hat design' is the wordplay.
'rewrites' indicates anagramming the letters.
'article' becomes 'an' (English indefinite article).
'about' means one lot of letters goes inside another.
'design' indicates an anagram.
'hat' is an anagram of 'tha'.
'an' enclosing 'tha' is 'athan'.
'evil' anagrammed gives 'levi'.
'levi'+'athan'='LEVIATHAN'
(Other definitions for leviathan that I've seen before include "An evil hat turns to anything of great size" , "Huge (sea) monster" , "Monstrous beast" , "Autocratic monarch" , "Gigantic" .)