Hitchhiker's digit (5)
I believe the answer is:
thumb
'hitchhiker's digit' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for thumb that I've seen before include "Tom ___ , traditional hero of English folklore" , "Hitch-hiker's tool" , "Opposable digit" , "Hitchhike - ..... a lift" , "Second part of 12" .)