An old army measure of length (5)
I believe the answer is:
cubit
'length' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.
'an old army measure' is the wordplay.
'an old army' becomes 'cu' (I can't justify this - if you can you should give a lot more credence to this answer).
'measure' becomes 'bit' (bit is a kind of measure).
'cu'+'bit'='CUBIT'
'of' is the link.
Can you help me to learn more?
(Other definitions for cubit that I've seen before include "distance" , "Biblical measure of length" , "Biblical length of approximately a forearm" , "Old measure of length (from elbow to fingertips)" , "two such might have been in the yard" .)