Patriarch begins investigating some American alternative cults (5)
I believe the answer is:
isaac
'patriarch' is the definition.
(biblical patriarch)
'begins investigating some american alternative cults' is the wordplay.
'begins' says to take the initial letters.
The initial letters of 'investigating some american alternative cults' is 'ISAAC'.
(Other definitions for isaac that I've seen before include "Father of 21" , "Newton's first name" , "Father to Jacob and Esau" , "& 17 Down SF author" , "boy almost meeting early death" .)