Crossword Genius

Digit with its own rule? (5)

Ross

I believe the answer is:

thumb

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

'digit' is the definition.
(I know that thumb is a type of finger)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for thumb that I've seen before include "Quickly turn pages" , "Hitch-hiker's tool" , "Digit that enables grasping" , "Try to get a lift" , "One opposed to other manual workers" .)

I've seen this clue in The Mirror and The Daily Express.
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