A number set about forming principles (6)
I believe the answer is:
tenets
'principles' is the definition.
(a tenet is a principle or belief)
'a number set about' is the wordplay.
'a number' becomes 'ten' (ten is a number).
'about' indicates anagramming the letters.
'set' is an anagram of 'ets'.
'ten'+'ets'='TENETS'
'forming' is the link.
(Other definitions for tenets that I've seen before include "Doctrines held to be true" , "Convictions" , "Principles of belief" , "Doctrines of belief" , "We stand by these" .)