Say what to do to get that whole number? (5)

I believe the answer is:

digit

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'that whole 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 "Single numeral" , "Need a bit of a hand" , "One's on hand" , "Finger; numeral" , "one of several at hand" .)

I've seen this clue in The Guardian.

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