Crossword Genius

One used in counting up to ten? (5)

Ross

I believe the answer is:

digit

I'm a little stuck... Click here to teach me more about this clue!

'one' is the definition.
The answer and definition can be both quantities as well as being singular nouns.
Maybe there's an association between them I don't understand?

'used in counting up to ten?' is the wordplay.
'used in' indicates the central letters (the letters used on the inside of the word).
'counting' becomes 'adding' (adding is a kind of counting).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'to ten?' becomes 'tig' (I can't justify this - if you can you should believe this answer much more).
The middle letters of 'adding' are 'di'.
'tig' in reverse letter order is 'git'.
'di'+'git'='DIGIT'

Can you help me to learn more?

(Other definitions for digit that I've seen before include "using 0 or 1 for example" , "that's on hand" , "A small number" , "Finger, thumb or toe" , "finger, perhaps" .)

I've seen this clue in the Evening Standard.
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