Pursue mum's doctrine (5)
I believe the answer is:
dogma
'doctrine' is the definition.
(dogma is a kind of doctrine)
'pursue mum's' is the wordplay.
'pursue' becomes 'dog' (dogging is a kind of pursuing).
'mum' becomes 'ma' (ma can mean mother).
'dog'+'ma'='DOGMA'
(Other definitions for dogma that I've seen before include "Doctrine accepted without question" , "Set of principles laid down as incontrovertibly true" , "Laid-down doctrine" , "Authoritative doctrine" , "Beliefs laid down as incontrovertibly true" .)