Doctrine in poem RC backed (5)
I believe the answer is:
credo
'doctrine' is the definition.
(I know that credo is a type of doctrine)
'poem rc backed' is the wordplay.
'poem' becomes 'ode' (ode is a kind of poem).
'backed' says the letters should be written backwards.
'ode'+'rc'='oderc'
'oderc' backwards is 'CREDO'.
'in' acts as a link.
(Other definitions for credo that I've seen before include "Statement of guiding aims" , "System of principles" , "Belief system" , "Decor (anag.)" , "Guiding beliefs" .)