Mark the sailor is a six-footer (6)
I believe the answer is:
scarab
'six-footer' is the definition.
(I know that scarab is a type of six-footer)
'mark the sailor' is the wordplay.
'mark' becomes 'scar' (mark left by an injury).
'the sailor' becomes 'ab' (abbreviation for able seaman).
'scar'+'ab'='SCARAB'
'is a' acts as a link.
(Other definitions for scarab that I've seen before include "Sacred Eygptian beetle" , "Beetle considered divine by ancient Egyptians" , "old lucky charm" , "insect" , "Beetle revered in ancient Egypt" .)