Crossword Genius

Young girl between six and nine initially (5)

Ross

I believe the answer is:

seven

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'young girl' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.

'six and nine initially' is the wordplay.
'six' becomes 'seve' (I am not sure about this - if you are sure you should believe this answer much more).
'and' says to put letters next to each other.
'initially' suggests taking the first letters.
The first letter of 'nine' is 'n'.
'seve'+'n'='SEVEN'

'between' is the link.

Can you help me to learn more?

(Other definitions for seven that I've seen before include "Wonders of the World - Deadly Sins" , "Cardinal" , "Numeral" , "Number of seas sailed?" , "Number (of stars in the sky?)" .)

I've seen this clue in the Evening Standard.
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