Principle essential to private network (5)
I believe the answer is:
tenet
'principle' is the definition.
(a tenet is a principle or value)
'essential to private network' is the wordplay.
'essential to' indicates a hidden word (letters in the inside or essence of the clue).
'TENET' can be found hidden inside 'private network'.
(Other definitions for tenet that I've seen before include "Article of belief" , "Item of belief" , "A belief or credo - it's a palindrome" , "Rule" , "Belief held as true" .)