Measure and cut out a big piece (5)
I believe the answer is:
cubit
'measure' is the definition.
(cubit is a kind of measure)
'cut out a big piece' is the wordplay.
'out' means to remove the last letter.
'a big piece' becomes 'bit' (bit is a kind of piece. I am not sure about the 'big' bit.).
'cut' with its final letter removed is 'cu'.
'cu'+'bit'='CUBIT'
'and' acts as a link.
Can you help me to learn more?
(Other definitions for cubit that I've seen before include "Get the measure" , "Biblical length of approximately a forearm" , "Ancient unit of length based on the length of the forearm" , "two such might have been in the yard" , "Elbow-to-finger measure" .)