Mark the sailor a six-footer (6)
I believe the answer is:
scarab
'a six-footer' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'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'
Can you help me to learn more?
(Other definitions for scarab that I've seen before include "divine thing once" , "Type of beetle" , "insect" , "Beetle regarded as divine by ancient Egyptians" , "Old abuse" .)