Princess with three siblings, mostly appropriate (4)
I believe the answer is:
anne
'princess with three siblings' is the definition.
(Princess Anne has three brothers)
'mostly appropriate' is the wordplay.
'mostly' means to remove the last letter.
'appropriate' becomes 'annex' (annexing is a kind of appropriating).
'annex' with its final letter removed is 'ANNE'.
(Other definitions for anne that I've seen before include "18th century Queen" , "Last Stuart monarch, d. 1714" , "First queen of Great Britain and Ireland" , "Stuart queen" , "--- Elliot, later Wentworth (J. Austen)" .)