Junk is not moving at bottom of stream (8)
I believe the answer is:
jettison
'junk' is the definition.
(thesaurus)
'is not moving at bottom of stream' is the wordplay.
'moving' indicates anagramming the letters.
'at bottom of' means one lot of letters go next to another (in a down clue, some letters go underneath others).
'stream' becomes 'jet'.
'is'+'not'='isnot'
'isnot' with letters rearranged gives 'tison'.
'tison' after 'jet' is 'JETTISON'.
(Other definitions for jettison that I've seen before include "'Throw away, cast overboard (8)'" , "21 [OFFLOAD]" , "bin" , "Chuck overboard" , "scrap" .)