Minx acting dreadfully, awfully mature initially (5)
I believe the answer is:
madam
'minx' is the definition.
'acting dreadfully awfully mature initially' is the wordplay.
'acting dreadfully' becomes 'mad' (I can't justify this - if you can you should believe this answer much more).
'initially' says to take the initial letters.
The initial letters of 'awfully mature' is 'am'.
'mad'+'am'='MADAM'
Can you help me to learn more?
(Other definitions for madam that I've seen before include "Formal address to a woman - precocious little girl" , "Woman running a brothel" , "Conceited girl (colloq.)" , "Palindrome used in speaking politely to a woman" , "'Call Me ---' (I. Berlin)" .)