Neural treatment is mythical (6)
I believe the answer is:
unreal
'mythical' is the definition.
(similar in meaning)
'neural treatment' is the wordplay.
'treatment' is an anagram indicator (the letters must be processed or treated).
'neural' anagrammed gives 'UNREAL'.
'is' acts as a link.
(Other definitions for unreal that I've seen before include "Fanciful" , "Not factual" , "Not actually existing" , "Lacking in substance" , "Dreamlike" .)