Ancient unit of length (5)
I believe the answer is:
cubit
'ancient unit of length' is the definition.
(I've seen this before)
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" .)