Not exactly cubic measure (5)
I believe the answer is:
cubit
'not exactly cubic measure' is the definition.
'cubit' can be an answer for 'measure' (cubit is a kind of measure). I'm not sure about the 'not exactly cubic' bit.
'cubic measure' is the wordplay.
'cubic' 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 "about eighteen inches" , "Ancient measure about the length of an arm" , "Old measure of length (from elbow to fingertips)" , "Old forearm measure" , "Biblical length of approximately a forearm" .)