One of a number? (5)

I believe the answer is:

digit

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

'a number?' is the definition.
(digit is a kind of whole number)

I can't explain the remainder of the clue.

(Other definitions for digit that I've seen before include "A character" , "A small number" , "zero, one, two, three, four, five, six, seven, eight, or nine" , "Need a bit of a hand" , "Finger; numeral" .)

I've seen this clue in the Evening Standard.

Want a hint initially instead of a full solution? Install my app