Sea debris (6)
I believe the answer is:
jetsam
'sea debris' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for jetsam that I've seen before include "Driftwood, wreckage" , "Ship's discharge" , "Unwanted stuff thrown overboard" , "Floating wreckage" , "Cargo deliberately thrown overboard to lighten ship" .)