Possibly sleeping around with old knight is wild (9)
I believe the answer is:
abandoned
'is wild' is the definition.
(I've seen this before)
'possibly sleeping around with old knight' is the wordplay.
'possibly sleeping' becomes 'abed' (I've seen this before).
'around' indicates putting letters inside.
'with' becomes 'and'.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'knight' becomes 'N' (abbreviation for knight in chess).
'and'+'o'+'n'='andon'
'abed' enclosing 'andon' is 'ABANDONED'.
(Other definitions for abandoned that I've seen before include "Deserted; uninhibited" , "Left - dissolute" , "Left forlorn" , "Obsolete" , "Profligate - gave up - left" .)