Mistakes at rare assembly (6)
I believe the answer is:
errata
'mistakes' is the definition.
(an erratum is a mistake in a printed work)
'at rare assembly' is the wordplay.
'assembly' indicates anagramming the letters.
'at'+'rare'='atrare'
'atrare' with letters rearranged gives 'ERRATA'.
(Other definitions for errata that I've seen before include "Publishing mistakes" , "List of errors" , "List of mistakes (in book)" , "List of printing mistakes" , "Mistakes in printed matter" .)