Sit on completed story (7)
I believe the answer is:
overlie
'sit on' is the definition.
(I've seen this in another clue)
'completed story' is the wordplay.
'completed' becomes 'over' (synonyms).
'story' becomes 'lie' (story can mean a fabrication).
'over'+'lie'='OVERLIE'
(Other definitions for overlie that I've seen before include "Create coincidence" , "Put in position superior to" , "Relievo (anag.)" , "Rest on top of" , "with crushing effect" .)