Greeting adopted in South America for a group of believers (4)
I believe the answer is:
shia
'a group of believers' is the definition.
(followers of Shia Islam)
'greeting adopted in south america' is the wordplay.
'greeting' becomes 'hi' (hi is a kind of greeting).
'adopted in' indicates putting letters inside (inserted letters are welcomed or adopted).
'south america' becomes 'SA' (abbreviation).
'hi' put within 'sa' is 'SHIA'.
'for' is the link.
(Other definitions for shia that I've seen before include "A major denomination of Islam" , "A main branch of Islam" , "A_main branch of Islam" , "One of the two main branches of orthodox Islam" .)