Opposable digit (5)
I believe the answer is:
thumb
'opposable digit' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for thumb that I've seen before include "Stubby digit?" , "Opposable digit" , "Hitch-hiking digit" , "Quickly turn pages" , "''Tom ..., the piper's son'' (nursery rhyme)" .)