Just why we hear story's a lie (5,4)
I believe the answer is:
fairy tale
'a lie' is the definition.
(thesaurus)
'just why we hear story's' is the wordplay.
'just' becomes 'fair' ('fair' is associated in meaning with 'just').
'we hear' shows a homophone (sound like).
'story' becomes 'tale' ('tale' can be a synonym of 'story').
'why' sounds like 'y'.
'fair'+'y'+'tale'='FAIRY TALE'
(Other definitions for fairy tale that I've seen before include "Children's magic tale or incredible story for adult" , "A magical story for children" , "''... of New York'' (The Pogues)" , "Children's story with magical beings" , "'Once upon a time...'" .)