The alteration having damaged, meant to repair the interior (9)
'the alteration' is the definition.
'damaged meant to repair the interior' is the wordplay.
'damaged' indicates an anagram.
'to repair' becomes 'mend' ('mend' can be a synonym of 'repair'**).
'the interior' means one lot of letters goes inside another.
'meant' anagrammed gives 'ament'.
'ament' going around 'mend' is 'AMENDMENT'.
'having' 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?", "Revision", "Correction".)