Ancient unit of length (5)
I believe the answer is:
cubit
'ancient unit of length' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for cubit that I've seen before include "Ancient linear measure" , "distance" , "Ancient unit of length based on the length of the forearm" , "Elbow-to-finger measure" , "about eighteen inches" .)