Mistakes upset a sailor in time (6)
I believe the answer is:
errata
'mistakes' is the definition.
(an erratum is a mistake in a printed work)
'upset a sailor in time' is the wordplay.
'upset' says the letters should be written backwards.
'a sailor' becomes 'tar' (tar can mean a sailor).
'in' means one lot of letters goes inside another.
'time' becomes 'era' (an era is a period of time).
'tar' backwards is 'rat'.
'rat' placed into 'era' is 'ERRATA'.
(Other definitions for errata that I've seen before include "Text mistakes" , "mistakes in print" , "List of mistakes (in book)" , "List of printing mistakes" , "List of text corrections to be made in a publication" .)