Follow mum in doctrine (5)
I believe the answer is:
dogma
'doctrine' is the definition.
(I know that dogma is a type of doctrine)
'follow mum' is the wordplay.
'follow' becomes 'dog' (dog can mean to follow or torment).
'mum' becomes 'ma' (both can mean mother).
'dog'+'ma'='DOGMA'
'in' acts as a link.
(Other definitions for dogma that I've seen before include "Set of principles laid down as incontrovertibly true" , "Some believe it certainly" , "Settled opinion" , "Code of belief" , "Inflexible principle" .)