Sailor going over fathom mark (6)
I believe the answer is:
target
'mark' is the definition.
(I know that mark is a class)
'sailor going over fathom' is the wordplay.
'sailor' becomes 'tar' (slang term for a sailor).
'going over' means one lot of letters go next to another.
'fathom' becomes 'get' (I've seen this in another clue).
'tar'+'get'='TARGET'
(Other definitions for target that I've seen before include "Aimpoint" , "Goal; victim" , "purpose" , "Goal, objective" , "Butt" .)