First found among most of these believers (7)
I believe the answer is:
theists
'believers' is the definition.
(theist is a kind of believer)
'first found among most of these' is the wordplay.
'first' becomes 'ist'.
'found among' indicates putting letters inside.
'most of' means to remove the last letter.
'these' with its final letter taken off is 'thes'.
'ist' placed within 'thes' is 'THEISTS'.
(Other definitions for theists that I've seen before include "Believers" , "people who believe in God" , "religious people" .)