Too often changed comment at bottom of page (8)
I believe the answer is:
footnote
'comment at bottom of page' is the definition.
(I've seen this before)
'too often changed' is the wordplay.
'changed' is an anagram indicator.
'too'+'often'='toooften'
'toooften' anagrammed gives 'FOOTNOTE'.
(Other definitions for footnote that I've seen before include "addition to manuscript" , "Too often at bottom of page" , "Text explanation" , "Text at page end" , "Comment at bottom of page" .)