Belief mother is after spaniel, say (5)
I believe the answer is:
dogma
'belief' is the definition.
(synonyms)
'mother is after spaniel say' is the wordplay.
'mother' becomes 'ma' (ma can mean mother).
'is after' is a charade indicator (letters next to each other).
'spaniel say' becomes 'dog' (spaniel is a kind of dog).
'ma' after 'dog' is 'DOGMA'.
(Other definitions for dogma that I've seen before include "Laid-down doctrine" , "Blind faith required" , "Body of religious doctrine" , "Laid down set of (religious) principles" , "Inflexible principles" .)