Mother chasing pet in principle (5)
I believe the answer is:
dogma
'principle' is the definition.
(thesaurus)
'mother chasing pet' is the wordplay.
'mother' becomes 'ma' (ma is a kind of mother).
'chasing' says to put letters next to each other.
'pet' becomes 'dog'.
'ma' put after 'dog' is 'DOGMA'.
'in' acts as a link.
(Other definitions for dogma that I've seen before include "Non-evidence based opinion" , "Set of beliefs" , "Go mad (anag.)" , "Item of belief" , "Firm doctrine" .)