Mistakes a deserter will make in time (6)
I believe the answer is:
errata
'mistakes' is the definition.
(an erratum is a mistake in a printed work)
'a deserter will make in time' is the wordplay.
'a deserter will make' becomes 'rat' (I can't justify this - if you can you should give a lot more credence to this answer).
'in' indicates putting letters inside.
'time' becomes 'era' (era is a kind of time).
'rat' going within 'era' is 'ERRATA'.
Can you help me to learn more?
(Other definitions for errata that I've seen before include "List of misprints" , "Text mistakes" , "Mistakes in printed work" , "Misprints in a book" , "Mistakes in printed proofs" .)