Embarrassing display by old boy, indecent (7)
I believe the answer is:
obscene
'indecent' is the definition.
(similar in meaning)
'embarrassing display by old boy' is the wordplay.
'embarrassing display' becomes 'scene' (I've seen this in another clue).
'by' 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 "off colour?" , "Rude" , "Pseudonym" , "One's CBE (anag)" , "unsuitable for children?" .)