Conspicuous vessel in the Mediterranean (6)
I believe the answer is:
marked
'conspicuous' is the definition.
(similar in meaning)
'vessel in the mediterranean' is the wordplay.
'vessel' becomes 'ark' (ark is a kind of vessel).
'in the' is an insertion indicator.
'mediterranean' becomes 'med' (short for Mediterranean Sea).
'ark' inserted inside 'med' is 'MARKED'.
(Other definitions for marked that I've seen before include "Singled out; clearly noticeable" , "Read, corrected and awarded a grade" , "Took good notice of - . . . . . . my words?" , "showing flag maybe" , "Pronounced" .)