Outfit — number for a future queen (6)
I believe the answer is:
kitten
'a future queen' is the definition.
(queen can mean a female cat)
'outfit number' is the wordplay.
'outfit' becomes 'kit' (kitting is a kind of outfiting).
'number' becomes 'ten' (ten is a kind of whole number).
'kit'+'ten'='KITTEN'
'for' acts as a link.
(Other definitions for kitten that I've seen before include "Immature cat" , "Little mewing ball of fur?" , "Little feline" , "Young cat, ferret etc." , "Baby cat" .)