Time conceals a deserter's mistakes (6)
I believe the answer is:
errata
'mistakes' is the definition.
(an erratum is a mistake in a printed work)
'time conceals a deserter's' is the wordplay.
'time' becomes 'era' (an era is a period of time).
'conceals' is an insertion indicator.
'a deserter' becomes 'rat' (both can mean a traitor).
'era' going around 'rat' is 'ERRATA'.
(Other definitions for errata that I've seen before include "Noted mistakes" , "mistakes in print" , "List of mistakes (in book)" , "List of misprints" , "List of errors" .)