Measure the big piece cut out (5)
I believe the answer is:
cubit
'measure' is the definition.
(cubit is a kind of measure)
'big piece cut out' is the wordplay.
'big piece' becomes 'bi' (I am not sure about this - if you are sure you should believe this answer much more).
'out' means one lot of letters goes inside another.
'bi' going inside 'cut' is 'CUBIT'.
'the' acts as a link.
Can you help me to learn more?
(Other definitions for cubit that I've seen before include "Old measure of length (from elbow to fingertips)" , "distance" , "Ancient unit of length based on the length of the forearm" , "Get the measure" , "Biblical measure of length" .)