Stone, almost identical, that is found on the beach (6)
I believe the answer is:
Here is my best explanation:
'that is found on the beach' is the definition.
'stone almost identical' is the wordplay.
'stone' becomes 'jet' (I have seen 'Black stone ' mean 'jet' so perhaps 'stone' could also mean 'jet').
'almost' means to remove the last letter (most of the word but not all of it).
'identical' becomes 'same' ('same' can be similar in meaning to 'identical').
'same' with its last letter taken away is 'sam'.
(Other definitions for jetsam that I've seen before include "Unwanted material thrown overboard", "Goods thrown into the sea", "lost cargo", "abandoned cargo", "What's thrown out", "Stuff thrown out", "Cargo cast overboard", "Goods jettisoned from a ship and washed up on shore", "debris on beach" and "Things thrown overboard and washed up ashore".)