Number new, not yet developed (6-3)
I believe the answer is:
twenty-one
'number' is the definition.
(twenty-one is a kind of number)
'new not yet developed' is the subsidiary indication.
'developed' indicates an anagram.
'new'+'not'+'yet'='newnotyet'
'newnotyet' with letters rearranged gives 'twentyone'.
(Other definitions for twenty-one that I've seen before include "XXI - used to be the coming of age year" , "Target total in blackjack or pontoon" , "Card game, blackjack or pontoon" .)