A number mostly belonging to us (4)
I believe the answer is:
four
'a number' is the definition.
(four is a kind of number)
'mostly belonging to us' is the wordplay.
'mostly' becomes 'f' (I can't justify this - if you can you should believe this answer much more).
'belonging to us' becomes 'our' (us, you and me).
'f'+'our'='FOUR'
Can you help me to learn more?
(Other definitions for four that I've seen before include "Square number" , "Cardinal" , "teatime?" , "An even number" , "Petit --, fancy biscuit" .)