Too often mistaken for an unimportant event (8)
I believe the answer is:
footnote
'an unimportant event' is the definition.
(a footnote in history)
'too often mistaken' is the wordplay.
'mistaken' indicates an anagram.
'too'+'often'='toooften'
'toooften' is an anagram of 'FOOTNOTE'.
'for' is the link.
(Other definitions for footnote that I've seen before include "Minor reference" , "Too often (anag.)" , "addition to manuscript" , "Comment at bottom of page" , "Explanation at bottom of page" .)