Shocking outburst by old boy (7)
I believe the answer is:
obscene
'shocking' is the definition.
(I've seen this before)
'outburst by old boy' is the wordplay.
'outburst' becomes 'scene' (as in making a scene in public).
'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 "not decent" , "coarse!" , "blue" , "Lewd or loathsome" , "Offensive - indecent" .)