Blue, old boy on stage (7)

I believe the answer is:

obscene

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'blue' is the definition.
(thesaurus)

'old boy on stage' is the wordplay.
'old boy' becomes 'ob'.
'on stage' becomes 'scene' (I can't justify this - if you can you should believe this answer much more).
'ob'+'scene'='OBSCENE'

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(Other definitions for obscene that I've seen before include "offending sensibilities?" , "Prurient" , "Indecent, lewd" , "Rude" , "Improper" .)

I've seen this clue in The Telegraph.

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