Measure a youngster for it (5)
I believe the answer is:
cubit
'measure' is the definition.
(cubit is a kind of measure)
'a youngster for it' is the wordplay.
'a youngster' becomes 'cub' (I've seen this in other clues).
'for' means one lot of letters go next to another.
'cub'+'it'='CUBIT'
(Other definitions for cubit that I've seen before include "Measure of length by which Noah's ark was described" , "Ancient linear measure" , "about eighteen inches" , "Ancient unit of length based on the length of the forearm" , "Biblical measure of length" .)