Disgusting old boy with part in play (7)
I believe the answer is:
obscene
'disgusting' is the definition.
(obscene can mean disgusting or indecent)
'old boy with part in play' is the wordplay.
I cannot really see how this works, but
'old' could be 'o' (abbreviation) and 'o' is located in the answer.
'boy' could be 'b' (abbreviation used in schools) and 'b' is found within the answer.
'part' could be 'scene' (I have seen 'Part of play ' mean 'scene' so perhaps 'part' could also mean 'scene') and 'scene' is present in the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for obscene that I've seen before include "Repugnant; indecent" , "Indecent, lewd" , "Bawdy" , "offending sensibilities?" , "Dirty" .)