On principle it's a canine parent (5)
I believe the answer is:
dogma
'on principle' is the definition.
'dogma' can be an answer for 'principle' (I've seen this before). I am not sure about the 'on' bit.
'a canine parent' is the wordplay.
'a canine' becomes 'dog' (dog is a kind of canine).
'parent' becomes 'ma' (ma can mean mum or mother).
'dog'+'ma'='DOGMA'
'it's' acts as a link.
Can you help me to learn more?
(Other definitions for dogma that I've seen before include "Authoritative doctrine" , "Opinion viewed as truth?" , "Body of creed" , "church teaching" , "Laid-down doctrine" .)