Old part of play bishop held to be indecent (7)
I believe the answer is:
obscene
'indecent' is the definition.
('obscene' can be similar in meaning to 'indecent')
'old part of play bishop held' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'part of play' becomes 'scene' ('scene' is part of 'play').
'bishop' becomes 'b' (abbreviation used in chess).
'held' means one lot of letters goes inside another.
'o'+'scene'='oscene'
'oscene' placed around 'b' is 'OBSCENE'.
'to be' acts as a link.
(Other definitions for obscene that I've seen before include "Salacious" , "Indecent or pornographic" , "Indecent, lewd" , "off colour?" , "Offensive - indecent" .)