I'm not sure a sailor lies about mistakes (6)
I believe the answer is:
errata
'mistakes' is the definition.
(an erratum is a mistake in a printed work)
'i'm not sure a sailor lies about' is the wordplay.
'i'm not sure' becomes 'er' (expression of hesitation).
'a sailor lies' becomes 'rat' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'about' becomes 'a' (abbreviation in Chambers).
'er'+'rat'+'a'='ERRATA'
Can you help me to learn more?
(Other definitions for errata that I've seen before include "Corrigenda" , "Text mistakes" , "Noted mistakes" , "List of printing mistakes" , "Misprints in a book" .)