Tale of Troy's undoing (5)
I believe the answer is:
story
'tale' is the definition.
('story' can be a synonym of 'tale')
'troy's undoing' is the wordplay.
'undoing' indicates anagramming the letters.
'troys' with letters rearranged gives 'STORY'.
'of' acts as a link.
(Other definitions for story that I've seen before include "Narrative or account" , "Tale - lie" , "Anecdote" , "See 10 and 19" , "Account, tale" .)