Moral member of royal family, mostly one quietly guided (10)
I believe the answer is:
principled
'moral' is the definition.
(similar in meaning)
'member of royal family mostly one quietly guided' is the wordplay.
'member of royal family' becomes 'prince' (prince is a type of royal).
'mostly' means to remove the last letter (most but not all of the word).
'one' becomes 'i' (Roman numeral).
'quietly' becomes 'p' (p for piano and quiet).
'guided' becomes 'led' ('lead' can be a synonym of 'guide').
'prince' with its final letter taken away is 'princ'.
'princ'+'i'+'p'+'led'='PRINCIPLED'
(Another definition for principled that I've seen is " With justification".)