Rodent following piper's rear in simple story (9)
I believe the answer is:
narrative
'story' is the definition.
('narrative' can be a synonym of 'story')
'rodent following piper's rear in simple' is the wordplay.
'rodent' becomes 'rat' (rat is a kind of rodent).
'following' says to put letters next to each other.
'rear' says to take the final letters (the back of the word).
'in' indicates putting letters inside.
'simple' becomes 'naive' ('naive' can be similar in meaning to 'simple'**).
The final letter of 'piper' is 'r'.
'rat' after 'r' is 'rrat'.
'rrat' put inside 'naive' is 'NARRATIVE'.
(Other definitions for narrative that I've seen before include "Chronicle" , "Spoken account; story" , "relation" , "Account of connected events, a story" , "Verbal account" .)