Involved in a cult that's crazy (7)
I believe the answer is:
lunatic
'crazy' is the definition.
(lunatic can mean crazy or mad)
'involved in a cult' is the wordplay.
'involved' indicates an anagram (involved can mean confusing or complex).
'in'+'a'+'cult'='inacult'
'inacult' anagrammed gives 'LUNATIC'.
'that's' acts as a link.
(Other definitions for lunatic that I've seen before include "Crackers or nuts" , "Madman - idiot" , "Mad" , "Type of fringe" , "nut" .)