Principles set down as incontrovertibly true (6)
I believe the answer is:
dogmas
'principles set down as incontrovertibly true' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for dogmas that I've seen before include "Set of principles set down as incontrovertibly true" , "Doctrines to be accepted without question" , "Doctrines put forward by church" .)