Unwise being disrespectful about king (9)
I believe the answer is:
imprudent
'unwise' is the definition.
('imprudent' can be similar in meaning to 'unwise')
'disrespectful about king' is the wordplay.
'disrespectful' becomes 'impudent' ('impudent' can be similar in meaning to 'disrespectful').
'about' indicates putting letters inside.
'king' becomes 'r' (r is abbreviation for rex).
'impudent' going around 'r' is 'IMPRUDENT'.
'being' acts as a link.
(Other definitions for imprudent that I've seen before include "Without forethought" , "Unwise" , "Reckless" , "Rash, lacking discretion" , "not wise" .)