Narrate the end of the story on the box (5)
I believe the answer is:
telly
'the box' is the definition.
(I've seen this before)
'narrate the end of the story on' is the wordplay.
'narrate' becomes 'tell' ('tell' can be a synonym of 'narrate').
'the end of' indicates one should take the final letters.
'on' is a charade indicator (letters next to each other).
The final letter of 'story' is 'y'.
'tell'+'y'='TELLY'
(Other definitions for telly that I've seen before include "- - Savalas" , "'The box' (colloquial)" , "The goggle-box" , "Set" , "cop in seventies" .)