Belief, first of all, can really enhance every day (5)
I believe the answer is:
creed
'belief' is the definition.
(I know that creed is a type of doctrine)
'first of all can really enhance every day' is the wordplay.
'first of all' says to take the initial letters.
'day' becomes 'd' (abbreviation for day).
The initial letters of 'can really enhance every' is 'cree'.
'cree'+'d'='CREED'
(Other definitions for creed that I've seen before include "System of principles or beliefs" , "System of religious principles" , "System of religious belief" , "Set of beliefs or principles" , "Religious doctrine" .)