Old length of copper gnawed (5)
I believe the answer is:
cubit
'old length' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'copper gnawed' is the wordplay.
'copper' becomes 'Cu' (chemical symbol).
'gnawed' becomes 'bit' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'cu'+'bit'='CUBIT'
'of' is the link.
Can you help me to learn more?
(Other definitions for cubit that I've seen before include "Ancient linear measure" , "Biblical length of approximately a forearm" , "Get the measure" , "about eighteen inches" , "Ancient measure about the length of an arm" .)