Mistakes awfully rare by army (6)
I believe the answer is:
errata
'mistakes' is the definition.
(erratum is a kind of mistake)
'awfully rare by army' is the wordplay.
'awfully' is an anagram indicator.
'by' means one lot of letters go next to another.
'army' becomes 'ta' (abbreviation for Territorial Army).
'rare' with letters rearranged gives 'erra'.
'erra'+'ta'='ERRATA'
(Other definitions for errata that I've seen before include "List of printed mistakes" , "Boobs in a book" , "Mistakes in printing or writing" , "Mistakes in printed material" , "Slips" .)