Mature king ordered mistake put down (7)
I believe the answer is:
erratum
'mistake put down' is the definition.
'erratum' can be an answer for 'mistake' (I've seen this before). I am not sure about the 'put down' bit.
'mature king ordered' is the wordplay.
'king' becomes 'R' (abbreviation for rex, king in Latin).
'ordered' indicates anagramming the letters.
'mature'+'r'='maturer'
'maturer' with letters rearranged gives 'ERRATUM'.
Can you help me to learn more?
(Other definitions for erratum that I've seen before include "fluff on paper" , "Mistake in book, etc" , "Mistake by Latin scholar" , "Misprint" , "Faux pas" .)