Initially trust every new space visitor's principle (5)
I believe the answer is:
tenet
'principle' is the definition.
(a tenet is a principle or value)
'initially trust every new space visitor's' is the wordplay.
'initially' indicates taking the first letters.
'new space visitor's' becomes 'net' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
The initial letters of 'trust every' is 'te'.
'te'+'net'='TENET'
Can you help me to learn more?
(Other definitions for tenet that I've seen before include "Belief - maxim" , "Canon" , "A religious doctrine proclaimed as true without proof" , "A belief or creed - a palindrome" , "A palindromic doctrine or belief" .)