An antediluvian measure (5)
I believe the answer is:
cubit
'an antediluvian measure' is the definition.
'cubit' can be an answer for 'measure' (cubit is a kind of measure). I'm not certain of the 'an antediluvian' bit.
'antediluvian measure' is the wordplay.
'antediluvian' becomes 'cu' (I can't justify this - if you can you should believe this answer much more).
'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" , "distance" , "about eighteen inches" , "Ancient linear measure" , "Measurement based on the length of the forearm" .)