Survey youngster mounted causes outrage (7)
I believe the answer is:
scandal
'outrage' is the definition.
('scandal' can be a synonym of 'outrage')
'survey youngster mounted' is the wordplay.
'survey' becomes 'scan' (I've seen this before).
'youngster' becomes 'lad' (I've seen this before).
'mounted' says the letters should be written backwards.
'lad' backwards is 'dal'.
'scan'+'dal'='SCANDAL'
'causes' is the link.
(Other definitions for scandal that I've seen before include "Malicious gossip" , "Malicious talk; outrage" , "Outraging event" , "Event causing public outrage" , "Outrageous event" .)