In a cult, turning psychotic (7)
I believe the answer is:
lunatic
'psychotic' is the definition.
(thesaurus)
'in a cult turning' is the wordplay.
'turning' indicates anagramming the letters.
'in'+'a'+'cult'='inacult'
'inacult' anagrammed gives 'LUNATIC'.
(Other definitions for lunatic that I've seen before include "Nutcase" , "Mad" , "Madman - idiot" , "bonkers" , "fruitcake" .)