Youngster left to cause confusion (5)
I believe the answer is:
babel
'confusion' is the definition.
(babel is a kind of confusion)
'youngster left' is the wordplay.
'youngster' becomes 'babe' (I've seen this before).
'left' becomes 'l' (common abbreviation).
'babe'+'l'='BABEL'
'to cause' is the link.
(Other definitions for babel that I've seen before include "Confused noise of many voices" , "Confused noise - biblical tower" , "Biblical tower location" , "Confused hubbub" , "Place of cacophony" .)