An old army measure? (5)
I believe the answer is:
cubit
'an old army measure?' is the definition.
'cubit' can be an answer for 'measure?' (cubit is a kind of measure). I am not certain of the 'an old army' bit.
'old army measure?' is the wordplay.
'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'
Can you help me to learn more?
(Other definitions for cubit that I've seen before include "Ancient unit of length based on the length of the forearm" , "Biblical length of approximately a forearm" , "distance" , "two such might have been in the yard" , "Biblical measure of length" .)