Unwise to be rude about king (9)
I believe the answer is:
imprudent
'unwise' is the definition.
(similar in meaning)
'rude about king' is the wordplay.
'rude' becomes 'impudent' (impudent means rude or disrespectful).
'about' indicates putting letters inside.
'king' becomes 'R' (abbreviation for rex, king in Latin).
'impudent' placed around 'r' is 'IMPRUDENT'.
'to be' is the link.
(Other definitions for imprudent that I've seen before include "Reckless" , "not wise" , "Rash, lacking discretion" , "Lacking in good sense or caution" , "Incautious" .)