Key underpinning native American belief system (5)
I believe the answer is:
creed
'belief system' is the definition.
'key underpinning native american' is the wordplay.
'key' becomes 'd' (I've seen this in other clues).
'underpinning' says to put letters next to each other (I've seen this in other clues).
'native american' becomes 'cree' (an indigenous people of North America**).
'd' after 'cree' is 'CREED'.
(Other definitions for creed that I've seen before include "System of religious principles" , "Religious doctrine" , "Catechism" , "Summary of articles of religious belief" , "teleprinter" .)