Story penned by an outsider (5)
I believe the answer is:
alien
'outsider' is the definition.
(thesaurus)
'story penned by an' is the wordplay.
'story' becomes 'lie' (I've seen this before).
'penned by' means one lot of letters goes inside another.
'lie' put within 'an' is 'ALIEN'.
(Other definitions for alien that I've seen before include "One is from outer space, perhaps" , "Foreign - film directed by Ridley Scott" , "Foreigner, outsider" , "24a [ESTRANGE]" , "Clanger?" .)