Crossword Genius

Restrain part of hand that is a bit redundant (5,5)

Ross

I believe the answer is:

check digit

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'hand that is a bit redundant' is the definition.
I don't know anything about this answer so I cannot tell whether it can be defined by this definition.

'restrain part' is the wordplay.
'restrain' becomes 'check' (synonyms).
'part' becomes 'digit' (digit is a kind of part).
'check'+'digit'='CHECK DIGIT'

'of' is the link.

Can you help me to learn more?

(Another definition for check digit that I've seen is " redundant element".)

I've seen this clue in The Times.
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