Old measure of copper coin (5)
I believe the answer is:
cubit
'old measure' is the definition.
(I've seen this before)
'copper coin?' is the wordplay.
'copper' becomes 'cu' (chemical symbol).
'coin?' becomes 'bit' (term for various historical coins).
'cu'+'bit'='CUBIT'
'of' is the link.
(Other definitions for cubit that I've seen before include "two such might have been in the yard" , "Measure of length by which Noah's ark was described" , "Measurement based on the length of the forearm" , "Biblical length of approximately a forearm" , "Old forearm measure" .)