Mistakes are overturned to catch traitor (6)
I believe the answer is:
errata
'mistakes' is the definition.
(an erratum is a mistake in a printed work)
'are overturned to catch traitor' is the wordplay.
'overturned' shows that the letters should be reversed in order.
'to catch' means one lot of letters goes inside another.
'traitor' becomes 'rat' (rat can mean a treacherous person).
'are' reversed gives 'era'.
'era' enclosing 'rat' is 'ERRATA'.
(Other definitions for errata that I've seen before include "List of mistakes (in book)" , "Misprints in a book" , "Boobs in a book" , "Mistakes in printing or writing" , "Mistakes in printed work" .)