Hand part with its own rule (5)
I believe the answer is:
thumb
'hand part' is the definition.
'thumb' can be an answer for 'part' (thumb is a kind of part). I am not sure about the 'hand' bit.
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for thumb that I've seen before include "Digit (used for making a hitch?)" , "Travel by getting free rides from motorists" , "Tom ___ , traditional hero of English folklore" , "Hitch-hiking digit" , "First digit" .)