Upset Ant and Dec with extremely offensive story (8)
I believe the answer is:
anecdote
'story' is the definition.
(thesaurus)
'upset ant and dec with extremely offensive' is the wordplay.
'upset' indicates an anagram.
'and' says to put letters next to each other (I've seen this in other clues).
'with' means one lot of letters go next to another.
'extremely' suggests removing the centre.
'offensive' with its centre removed is 'oe'.
'dec' after 'oe' is 'oedec'.
'ant' after 'oedec' is 'oedecant'.
'oedecant' with letters rearranged gives 'ANECDOTE'.
(Other definitions for anecdote that I've seen before include "Neat code (anag.)" , "A short tale, usually true" , "Short accoung of an incident" , "One acted (anag) - amusing reminiscence" , "Short narration" .)