The leader of the clergy in established religious group (4)
I believe the answer is:
sect
'religious group' is the definition.
(eg a religious sect)
'the leader of the clergy in established' is the wordplay.
'the leader of' indicates taking the first letters.
'in' is an insertion indicator.
'established' becomes 'set' (set can mean established or agreed upon).
The initial letter of 'clergy' is 'c'.
'c' going into 'set' is 'SECT'.
(Other definitions for sect that I've seen before include "Dissenting body" , "Subdivision of a religion" , "Nonconformist faction" , "party" , "Breakaway group" .)