Religious people back American (6)
I believe the answer is:
hindus
'religious people' is the definition.
(I've seen this before)
'back american' is the wordplay.
'back' becomes 'hind' (synonyms).
'american' becomes 'US' (United States).
'hind'+'us'='HINDUS'
(Other definitions for hindus that I've seen before include "Members of great Indian religion" , "pundits?" , "Asians with faith" , "Religious people" , "Members of great religion in India" .)