Firm belief in man's best friend, mother (5)
I believe the answer is:
dogma
'firm belief' is the definition.
'man's best friend mother' is the wordplay.
'man's best friend' becomes 'dog'.
'mother' becomes 'ma' (ma is a kind of mother).
'dog'+'ma'='DOGMA'
'in' is the link.
(Other definitions for dogma that I've seen before include "Body of creed" , "fixed opinion" , "A religious belief" , "Inflexible principle" , "Firm doctrine" .)