Span of religion primarily found in a church (4)
I believe the answer is:
arch
'span' is the definition.
(thesaurus)
'religion primarily found in a church' is the wordplay.
'primarily' indicates taking the first letters.
'found in' is an insertion indicator.
'church' becomes 'ch' (common abbreviation).
The first letter of 'religion' is 'r'.
'a'+'ch'='ach'
'r' inserted into 'ach' is 'ARCH'.
'of' acts as a link.
(Other definitions for arch that I've seen before include "Waggish" , "Foremost" , "Cunning; bridge" , "Curved entrance" , "Clever" .)