Measure young Italian (5)
I believe the answer is:
cubit
'measure' is the definition.
(cubit is a kind of measure)
'young italian' is the wordplay.
'young' becomes 'cub' (cub is a kind of young).
'italian' becomes 'it' (abbreviation for Italian).
'cub'+'it'='CUBIT'
(Other definitions for cubit that I've seen before include "two such might have been in the yard" , "about eighteen inches" , "Biblical measure of length" , "Ancient unit of length based on the length of the forearm" , "distance" .)