Fuss after old boy's rude (7)
I believe the answer is:
obscene
'rude' is the definition.
(obscene can mean rude or indecent)
'fuss after old boy's' is the wordplay.
'fuss' becomes 'scene' (making a scene means making a fuss).
'after' says to put letters next to each other.
'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 "not decent" , "Bawdy" , "Indecent, lewd" , "Offensively vulgar" , "Rude" .)