Delinquent, one that's sadly dropped out (8)
I believe the answer is:
tearaway
'delinquent' is the definition.
(both can mean a naughty child)
'one that's sadly dropped out' is the wordplay.
'one that's sadly dropped' becomes 'tear' (teardrops are released when someone is sad).
'out' becomes 'away' (synonyms).
'tear'+'away'='TEARAWAY'
(Other definitions for tearaway that I've seen before include "rebel" , "Reckless and impetuous person" , "A tough" , "Yob" , "Reckless violent person" .)