Youngster carrying large blunt instrument (4)
I believe the answer is:
club
'blunt instrument' is the definition.
(I've seen this in another clue)
'youngster carrying large' is the wordplay.
'youngster' becomes 'cub' (cub can mean a young person).
'carrying' is an insertion indicator.
'large' becomes 'L' (eg in clothes sizes).
'cub' enclosing 'l' is 'CLUB'.
(Other definitions for club that I've seen before include "Good meeting-place" , "Union - strike" , "Association; cudgel" , "Kind of weapon or golf stick" , "Place for dance" .)