The first of the converts in established denomination (4)
I believe the answer is:
sect
'denomination' is the definition.
(I've seen this before)
'the first of the converts in established' is the wordplay.
'the first of' indicates taking the first letters.
'in' is an insertion indicator.
'established' becomes 'set' (set can mean established or agreed upon).
The first letter of 'converts' is 'c'.
'c' put inside 'set' is 'SECT'.
(Other definitions for sect that I've seen before include "Group, faction" , "Religious splinter group" , "Subdivision of a larger religious group" , "Religious faction" , "Subdivision of a religion" .)