Old way to introduce a popular decree (6)
I believe the answer is:
ordain
'decree' is the definition.
(ordaining is a kind of decreeing)
'old way to introduce a popular' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'way' becomes 'rd' (**).
'to introduce' means to remove the last letter (I've seen 'introducing' mean this).
'a' becomes 'an'.
'popular' becomes 'in'.
'an' with its final letter taken away is 'a'.
'o'+'rd'+'a'+'in'='ORDAIN'
(Other definitions for ordain that I've seen before include "Officially order" , "Invest with priestly authority" , "Initiate as priest" , "Decree or appoint to priesthood" , "Make someone a clergyman" .)