Go mad, crackers, teaching (5)
I believe the answer is:
dogma
'teaching' is the definition.
(dogma is a kind of teaching)
'go mad crackers' is the wordplay.
'crackers' indicates anagramming the letters (informal term for mad).
'go'+'mad'='gomad'
'gomad' anagrammed gives 'DOGMA'.
(Other definitions for dogma that I've seen before include "Blind faith required" , "Code of belief" , "Laid down set of (religious) principles" , "Opinion viewed as truth?" , "Code of beliefs accepted as authoritative" .)