Fuss after old boy becomes offensive (7)
I believe the answer is:
obscene
'offensive' is the definition.
(similar in meaning)
'fuss after old boy' is the wordplay.
'fuss' becomes 'scene' (I can't justify this - if you can you should believe this answer much more).
'after' is a charade indicator (letters next to each other).
'old boy' becomes 'OB' (abbreviation for a private school 'old boy').
'scene' put after 'ob' is 'OBSCENE'.
'becomes' acts as a link.
Can you help me to learn more?
(Other definitions for obscene that I've seen before include "repellent" , "Offensive - indecent" , "not decent" , "Pseudonym" , "off colour?" .)