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' (young bear).
'carrying' is an insertion indicator.
'large' becomes 'L' (eg in clothes sizes).
'cub' placed around '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" .)