Cardinal imprisoning papal envoy is demoted (9)
I believe the answer is:
relegated
'is demoted' is the definition.
(I've seen this before)
'cardinal imprisoning papal envoy' is the wordplay.
'cardinal' becomes 'red' (I've seen this in other clues).
'imprisoning' means one lot of letters goes inside another.
'papal envoy' becomes 'legate' (I've seen this before).
'red' enclosing 'legate' is 'RELEGATED'.
(Other definitions for relegated that I've seen before include "Kicked downstairs" , "banished" , "Demoted" , "Put down" , "Downgraded" .)