Mistakes are holding sailor back (6)
I believe the answer is:
errata
'mistakes' is the definition.
(an erratum is a mistake in a printed work)
'are holding sailor back' is the wordplay.
'holding' indicates putting letters inside.
'sailor' becomes 'tar' (term for a sailor).
'back' is a reversal indicator.
'are' placed around 'tar' is 'atarre'.
'atarre' backwards is 'ERRATA'.
(Other definitions for errata that I've seen before include "Noted mistakes" , "Mistakes in printed material" , "Publishing mistakes" , "Boobs in a book" , "List of mistakes (in book)" .)