Crossword Genius

A number mostly belonging to us (4)

Ross

I believe the answer is:

four

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'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" .)

I've seen this clue in The Irish Independent.
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