Madman let loose in a cult (7)
I believe the answer is:
lunatic
'madman' is the definition.
('lunatic' can be a synonym of 'madman')
'let loose in a cult' is the wordplay.
'let loose' indicates anagramming the letters.
'in'+'a'+'cult'='inacult'
'inacult' anagrammed gives 'LUNATIC'.
(Other definitions for lunatic that I've seen before include "Imbecile" , "Madman - idiot" , "Mad" , "nut" , "bonkers" .)