Disturbance after old boy's rude (7)
I believe the answer is:
obscene
'rude' is the definition.
(obscene can mean rude or indecent)
'disturbance after old boy's' is the wordplay.
'disturbance' becomes 'scene' (I've seen this in another clue).
'after' means one lot of letters go next to another.
'old boy' becomes 'OB' (abbreviation for a private school 'old boy').
'scene' put after 'ob' is 'OBSCENE'.
(Other definitions for obscene that I've seen before include "Shocking" , "Disgusting, repugnant" , "Offensively vulgar" , "Abhorrent or lewd" , "Bawdy" .)