Mark Sailor is a beetle? (6)
I believe the answer is:
scarab
'a beetle?' is the definition.
(scarab is a kind of beetle)
'mark sailor' is the wordplay.
'mark' becomes 'scar' (synonyms).
'sailor' becomes 'ab' (abbreviation for able seaman).
'scar'+'ab'='SCARAB'
'is' acts as a link.
(Other definitions for scarab that I've seen before include "Sacred beetle of Eygpt" , "Large dark beetle" , "Ancient Egyptian symbol" , "Egyptian stone" , "Sacred beetle of ancient Egypt" .)