Doctrine in poem Roman Catholic rejected (5)
I believe the answer is:
credo
'doctrine' is the definition.
(credo is a kind of doctrine)
'poem roman catholic rejected' is the wordplay.
'poem' becomes 'ode' (ode is a kind of poem).
'roman catholic' becomes 'rc' (abbreviation for Roman Catholic).
'rejected' shows that the letters should be reversed in order.
'ode'+'rc'='oderc'
'oderc' written backwards gives 'CREDO'.
'in' is the link.
(Other definitions for credo that I've seen before include "Words to live by; belief" , "Set of beliefs about leftist circle" , "One's system of beliefs" , "Faith or belief" , "Section of mass" .)