Ten madmen surprisingly making an improvement (9)
I believe the answer is:
amendment
'an improvement' is the definition.
(I know that amendment is a type of correction)
'ten madmen surprisingly' is the wordplay.
'surprisingly' indicates an anagram (a surprising ordering of the letters).
'ten'+'madmen'='tenmadmen'
'tenmadmen' with letters rearranged gives 'AMENDMENT'.
'making' is the link.
(Other definitions for amendment that I've seen before include "Minor improvement" , "Statement added to or revising a document" , "the fifth one benefiting Uncle Sam's defendants?" , "Corrective treatment" , "Revision" .)