Vessel in the Mediterranean, that's obvious (6)
I believe the answer is:
marked
'obvious' is the definition.
(similar in meaning)
'vessel in the mediterranean' is the wordplay.
'vessel' becomes 'ark' (ark is a kind of vessel).
'in the' means one lot of letters goes inside another.
'mediterranean' becomes 'med' (short for Mediterranean Sea).
'ark' inserted inside 'med' is 'MARKED'.
'that's' is the link.
(Other definitions for marked that I've seen before include "Took good notice of - . . . . . . my words?" , "Read, corrected and awarded a grade" , "showing flag maybe" , "Singled out; clearly noticeable" , "Easily noticeable" .)