Doctrine of equal return (5)
I believe the answer is:
tenet
'doctrine' is the definition.
(tenet is a kind of doctrine)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for tenet that I've seen before include "Belief - maxim" , "Key principle" , "Teaching" , "That's the law" , "A palindromic doctrine or belief" .)