A youngster takes it to be an obsolete measure (5)
I believe the answer is:
cubit
'obsolete measure' is the definition.
'cubit' can be an answer for 'measure' (cubit is a kind of measure). I am not certain of the 'obsolete' bit.
'a youngster takes it' is the wordplay.
'a youngster' becomes 'cub' (cub can mean a young person).
'takes' means one lot of letters go next to another.
'cub'+'it'='CUBIT'
'to be an' is the link.
Can you help me to learn more?
(Other definitions for cubit that I've seen before include "Old forearm measure" , "Ancient linear measure" , "about eighteen inches" , "distance" , "Biblical length of approximately a forearm" .)