A sailor concerned with rejected list of mistakes (6)
I believe the answer is:
errata
'list of mistakes' is the definition.
(I've seen this before)
'a sailor concerned with rejected' is the wordplay.
'sailor' becomes 'tar' (slang term for a sailor).
'concerned with' becomes 're' (re can mean regarding, concerning).
'rejected' says the letters should be written backwards.
'a'+'tar'+'re'='atarre'
'atarre' in reverse letter order is 'ERRATA'.
(Other definitions for errata that I've seen before include "Mistakes in printed proofs" , "Mistakes in printed work" , "Mistakes in printed matter" , "List of printing mistakes" , "Slips" .)