Go mad formulating principle (5)
I believe the answer is:
dogma
'principle' is the definition.
(I know that dogma is a type of principle)
'go mad formulating' is the wordplay.
'formulating' indicates an anagram.
'go'+'mad'='gomad'
'gomad' anagrammed gives 'DOGMA'.
(Other definitions for dogma that I've seen before include "Principles laid down by authority as true" , "A religious belief" , "Go mad (anag.)" , "Item of belief" , "Set of principles laid down as incontrovertibly true" .)