It's said complete article finishes off lunatic fringe (9)
I believe the answer is:
'it's said' is the definition.
(an utterance is something which is said)
'complete article finishes off lunatic fringe' is the wordplay.
'complete' becomes 'utter' (I've seen this before).
'article' becomes 'an' (indefinite article).
'finishes off' suggests the final letters (take the finishing letters off of the words).
The final letters of 'lunatic fringe' are 'ce'.
(Other definitions for utterance that I've seen before include "Word" , "Statement" , "sermon, say" , "Remark" , "Something said" .)