Supercilious criminal going down (13)
I believe the answer is:
condescending
'supercilious' is the definition.
(I've seen this before)
'criminal going down' is the wordplay.
'criminal' becomes 'con' (abbreviation for 'convict').
'going down' becomes 'descending' ('descend' can be a synonym of 'go down').
'con'+'descending'='CONDESCENDING'
(Other definitions for condescending that I've seen before include "Snooty" , "Showing (to others) that one feels superior" , "Toffee-nosed" , "Offensively patronising" , "Disdainful" .)