Belief in honour? Leave it out, love! (5)
I believe the answer is:
credo
'belief in honour?' is the definition.
'credo' can be an answer for 'belief' (credo is a kind of belief). I'm not sure about the 'in honour?' bit.
'leave it out love' is the wordplay.
'leave it out' becomes 'cred' (I am not sure about this - if you are sure you should believe this answer much more).
'love' becomes 'o' (love means zero in tennis).
'cred'+'o'='CREDO'
Can you help me to learn more?
(Other definitions for credo that I've seen before include "Set of beliefs about leftist circle" , "System of principles" , "Formal statement of beliefs" , "Set of principles" , "A statement of one's beliefs" .)