Belief that a firm is out of the red (5)
I believe the answer is:
credo
'belief' is the definition.
(I know that credo is a type of doctrine)
'a firm is out of the red' is the wordplay.
'a firm' becomes 'co' (abbreviation for company).
'is out of' is an insertion indicator (some letters on the outside of others).
'co' enclosing 'red' is 'CREDO'.
'that' acts as a link.
(Other definitions for credo that I've seen before include "mission statement" , "Doctrine or belief" , "One's system of beliefs" , "I believe in Latin" , "music in church" .)