Mistakes awfully rare? Thank you! (6)
I believe the answer is:
errata
'mistakes' is the definition.
(erratum is a kind of mistake)
'awfully rare? thank you' is the wordplay.
'awfully' indicates an anagram.
'thank you' becomes 'ta'.
'rare' is an anagram of 'erra'.
'erra'+'ta'='ERRATA'
(Other definitions for errata that I've seen before include "List of printed mistakes" , "Noted mistakes" , "Slips" , "corrected mistakes" , "List of text corrections to be made in a publication" .)