Fundamental truth I place in member of royal family (9)
I believe the answer is:
Here is my best explanation:
'fundamental truth' is the definition.
(I've seen this before)
'i place in member of royal family' is the subsidiary indication.
'place' becomes 'pl' (I've seen this in other clues).
'in' means one lot of letters goes inside another.
'member of royal family' becomes 'prince' (I've seen this before).
'ipl' inserted inside 'prince' is 'principle'.
(Other definitions for principle that I've seen before include "Fundamental truth - moral rule", "Morally correct behaviour", "Personal rule of conduct", "set", "General guiding rule", "In which, theoretically", "Basic standard", "Law of nature, rule of conduct", "Tenet" and "Axiom".)