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 subsidiary indication.
'stone' becomes 'jet'.
'almost' means to remove the last letter.
'identical' becomes 'same' ('same' can be similar in meaning to 'identical').
'same' with its last letter taken off is 'sam'.
(Other definitions for jetsam that I've seen before include "Discarded objects washed ashore", "Floating wreckage", "Floating rubbish", "Driftwood, wreckage", "Unwanted stuff thrown overboard", "Unwanted material thrown overboard", "Stuff cast overboard", "Ship's cargo thrown overboard", "Cargo deliberately thrown overboard to lighten ship" and "Goods thrown overboard from ship in distress".)