Detestable bit of play written by old boy (7)
I believe the answer is:
obscene
'detestable' is the definition.
('obscene' can be a synonym of 'detestable')
'bit of play written by old boy' is the wordplay.
'bit of play' becomes 'scene' ('scene' is part of 'play').
'written by' means one lot of letters go next to another.
'old boy' becomes 'ob'.
'scene' put after 'ob' is 'OBSCENE'.
(Other definitions for obscene that I've seen before include "Rude" , "coarse!" , "X-rated" , "Prurient" , "Lewd - offensive" .)