Fundamental truth I place in member of royal family (9)
I believe the answer is:
principle
'fundamental truth' is the definition.
(I've seen this before)
'i place in member of royal family' is the wordplay.
'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).
'i'+'pl'='ipl'
'ipl' inserted within 'prince' is 'PRINCIPLE'.
(Other definitions for principle that I've seen before include "Morally correct behaviour" , "In which, theoretically" , "General guiding rule" , "Basic standard" , "Fundamental truth - moral rule" .)