6 down
Birds
in
most
of
the
riverside
plants
(8)
Birds in most of the riverside plants (8)
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The definition is at the beginning of the clue.
How the clue works
Right, so birds is your definition there — thrushes, the songbirds. For the wordplay, take "the" and drop its last letter, leaving you with TH, then bolt on RUSHES (your riverside plants) — TH plus RUSHES gives you the answer.