Belief is firm about shade (5)
I believe the answer is:
credo
'belief' is the definition.
(I know that credo is a type of doctrine)
'firm about shade' is the wordplay.
'firm' becomes 'co' (abbreviation for company).
'about' means one lot of letters goes inside another.
'shade' becomes 'red' (red is a colour or shade).
'co' placed around 'red' is 'CREDO'.
'is' acts as a link.
(Other definitions for credo that I've seen before include "Mass movement" , "conviction" , "Apostles' beliefs" , "Decor (anag.)" , "I believe in Latin" .)