One used in counting up to ten? (5)
I believe the answer is:
digit
'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" .)