Post written about obstacle knocked over in story (7)
I believe the answer is:
parable
'story' is the definition.
(I know that parable is a type of story)
'post written about obstacle knocked over' is the wordplay.
'post' becomes 'pale' (I've seen this before).
'written about' indicates putting letters inside.
'obstacle' becomes 'bar' (bar can mean an obstacle or blockage).
'knocked over' shows that the letters should be reversed in order.
'bar' back-to-front is 'rab'.
'pale' placed around 'rab' is 'PARABLE'.
'in' acts as a link.
(Other definitions for parable that I've seen before include "Story with ethical point" , "Instructive story" , "Moral tale" , "Story illustrating moral lesson" , "Moral-carrying story" .)