Jack, with mates at sea, discarded goods (6)
I believe the answer is:
jetsam
'discarded goods' is the definition.
(I've seen this before)
'jack with mates at sea' is the wordplay.
'jack' becomes 'J' (playing card abbreviation).
'with' says to put letters next to each other.
'at sea' indicates anagramming the letters.
'mates' is an anagram of 'etsam'.
'j'+'etsam'='JETSAM'
(Other definitions for jetsam that I've seen before include "Driftwood, wreckage" , "Unwanted material thrown overboard" , "Discarded rubbish" , "Goods washed ashore from shipwreck" , "Goods from ship washed ashore" .)