Crossword Genius

When it's up, it's OK (5)

Ross

I believe the answer is:

thumb

(Other definitions for thumb that I've seen before include "Travel by getting free rides from motorists" , "Hand digit" , "Try to get free ride" , "Digit (used for making a hitch?)" , "Travel by . . . . . - hitchhike" .)

I've seen this clue in The Guardian.
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