Finger with its own rule? (5)
I believe the answer is:
thumb
'finger' is the definition.
(I know that finger can be written as thumb)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for thumb that I've seen before include "Digit used in getting free rides from motorists" , "Short thick manual digit" , "Digit (used for making a hitch?)" , "could do it for a lift" , "Opposable digit" .)