Proof statement has been altered (9)
I believe the answer is:
testament
'proof' is the definition.
(a testament to something is proof of it)
'statement has been altered' is the wordplay.
'has been altered' indicates an anagram.
'statement' is an anagram of 'TESTAMENT'.
(Other definitions for testament that I've seen before include "with 1a [NEW] where is Mark?" , "(Person's) will" , "books, with 1a [OLD]" , "Biblical division" , "evidence" .)