A French vessel in sea, lacking identification (8)
I believe the answer is:
unmarked
'lacking identification' is the definition.
'a french vessel in sea' is the wordplay.
'a french' becomes 'un' ('a' in French).
'vessel' becomes 'ark' (ark is a kind of vessel).
'in' is an insertion indicator.
'sea' becomes 'med' (short for Mediterranean).
'ark' inserted into 'med' is 'marked'.
'un'+'marked'='UNMARKED'
(Other definitions for unmarked that I've seen before include "Not noticed" , "Without blemish" .)