Belief system is welcomed by those people (6)
I believe the answer is:
theism
'belief system' is the definition.
(I've seen this in another clue)
'is welcomed by those people' is the wordplay.
'welcomed by' indicates putting letters inside (letters welcomed inside).
'those people' becomes 'them' ('they' refers to a group of people).
'is' placed inside 'them' is 'THEISM'.
(Other definitions for theism that I've seen before include "of faith" , "Belief in a creator" , "Belief in existence of a God revealed to man" , "Belief in a deity" , "The doctrine or belief in the existence of a God or gods" .)