Jack behind Mark, crawler (6)
I believe the answer is:
scarab
'crawler' is the definition.
(I've seen this before)
'jack behind mark' is the wordplay.
'jack' becomes 'ab' (both can mean a sailor).
'behind' means one lot of letters go next to another.
'mark' becomes 'scar' (mark left by an injury).
'ab' put after 'scar' is 'SCARAB'.
(Other definitions for scarab that I've seen before include "Large dung beetle" , "Holy beetle" , "Beetle revered by ancient Egyptians" , "divine creature" , "crawler" .)