Mostly ill-mannered, I see, and unwise (9)
I believe the answer is:
impolitic
'unwise' is the definition.
(similar in meaning)
'mostly ill-mannered i see' is the wordplay.
'mostly' means to remove the last letter (most but not all of the word).
'ill-mannered' becomes 'impolite' (similar in meaning).
'i see' becomes 'ic' (I can't justify this - if you can you should give a lot more credence to this answer).
'impolite' with its last letter taken off is 'impolit'.
'impolit'+'ic'='IMPOLITIC'
'and' acts as a link.
Can you help me to learn more?
(Other definitions for impolitic that I've seen before include "Not prudent" , "Inexpedient" , "Ill-advised" , "Lacking prudence, unwise" .)