Mistakes unusually rare: thanks! (6)
I believe the answer is:
errata
'mistakes' is the definition.
(erratum is a kind of mistake)
'unusually rare thanks' is the wordplay.
'unusually' indicates an anagram.
'thanks' becomes 'ta' (UK informal version of 'thank you').
'rare' anagrammed gives 'erra'.
'erra'+'ta'='ERRATA'
(Other definitions for errata that I've seen before include "Misprints in a book" , "Printing corrections" , "List of errors" , "(Printing) errors" , "Publishing mistakes" .)