Follow mother in principle (5)
I believe the answer is:
dogma
'principle' is the definition.
(I know that dogma is a type of principle)
'follow mother' is the wordplay.
'follow' becomes 'dog' (dog can mean to follow or torment).
'mother' becomes 'ma' (Ma is another word for mother or mum).
'dog'+'ma'='DOGMA'
'in' acts as a link.
(Other definitions for dogma that I've seen before include "Doctrine accepted as true without proof" , "Laid down set of (religious) principles" , "Doctrine laid down with authority" , "Fixed belief" , "Article of faith" .)