Oriental in a tent, as a rule (5)
I believe the answer is:
tenet
'a rule' is the definition.
(I know that principle can be written as tenet)
'oriental in a tent' is the wordplay.
'oriental' becomes 'e'.
'in' is an insertion indicator.
'e' inserted within 'tent' is 'TENET'.
'as' acts as a link.
(Other definitions for tenet that I've seen before include "Conviction" , "A palindromic doctrine or creed" , "you may believe in it" , "Teaching" , "A religious doctrine proclaimed as true without proof" .)