In a cult, turning bonkers (7)
I believe the answer is:
lunatic
'bonkers' is the definition.
(I've seen this before)
'in a cult turning' is the wordplay.
'turning' is an anagram indicator.
'in'+'a'+'cult'='inacult'
'inacult' anagrammed gives 'LUNATIC'.
(Other definitions for lunatic that I've seen before include "Nutcase" , "bonkers" , "Harebrained" , "bananas" , "Mad" .)