Lout starts off yelling on bus (3)
I believe the answer is:
yob
'lout' is the definition.
(both can mean a rude or criminal young person)
'starts off yelling on bus' is the wordplay.
'starts off' suggests taking the first letters.
The first letters of 'yelling on bus' is 'YOB'.
(Other definitions for yob that I've seen before include "Troublemaker" , "Thug" , "Lout (sl.)" , "Hooligan, ruffian" , "Hooligan kind of boy" .)