Mark, a seaman, or a beetle (6)
I believe the answer is:
scarab
'a beetle' is the definition.
(scarab is a kind of beetle)
'mark a seaman' is the wordplay.
'mark' becomes 'scar' (mark left by an injury).
'a seaman' becomes 'ab' (short for able seaman).
'scar'+'ab'='SCARAB'
'or' is the link.
(Other definitions for scarab that I've seen before include "Old abuse" , "Beetle, sacred to the ancient Egyptians" , "Ancient Egyptian symbol" , "Large dung beetle" , "Sacred beetle of Eygpt" .)