Beetle mark sailor? (6)
I believe the answer is:
scarab
'beetle' is the definition.
(I know that scarab is a type of dung beetle)
'mark sailor?' is the wordplay.
'mark' becomes 'scar' (mark left by an injury).
'sailor?' becomes 'ab' (abbreviation for able seaman).
'scar'+'ab'='SCARAB'
(Other definitions for scarab that I've seen before include "Sacred Egyptian beetle" , "Large dark beetle" , "Beetle, sacred to the ancient Egyptians" , "Beetle considered divine by ancient Egyptians" , "Sacred beetle of ancient Egypt" .)