Madman flourished in a cult (7)
I believe the answer is:
lunatic
'madman' is the definition.
(I know that madman can be written as lunatic)
'flourished in a cult' is the wordplay.
'flourished' indicates an anagram (I've seen 'flourishing' mean this (letters grow or flourish differently)).
'in'+'a'+'cult'='inacult'
'inacult' with letters rearranged gives 'LUNATIC'.
(Other definitions for lunatic that I've seen before include "Foolish person" , "Crackers or nuts" , "Type of fringe" , "Insane person" , "Harebrained" .)