Little demon, not polite and no doubt extremely rash (9)
I believe the answer is:
imprudent
'rash' is the definition.
(similar in meaning)
'little demon not polite and no doubt extremely' is the wordplay.
'little demon' becomes 'imp' (I've seen this in another clue).
'not polite' becomes 'rude' ('rude' is the opposite to 'polite').
'and' means one lot of letters go next to another.
'no' becomes 'n' (common abbreviation).
'doubt extremely' becomes 't' (I am not sure about this - if you are sure you should believe this answer much more).
'imp'+'rude'+'n'+'t'='IMPRUDENT'
Can you help me to learn more?
(Other definitions for imprudent that I've seen before include "Incautious" , "'making the wrong choices'?" , "Reckless" , "Lacking in good sense or caution" , "Unwise" .)