Conspicuous vessel in the Mediterranean (6)
I believe the answer is:
marked
'conspicuous' is the definition.
('marked' can be similar in meaning to 'conspicuous')
'vessel in the mediterranean' is the wordplay.
'vessel' becomes 'ark' (ark is a kind of vessel).
'in the' indicates putting letters inside.
'mediterranean' becomes 'med' (I've seen this in other clues).
'ark' going within '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" .)