Floater __ on English river (7,4)

I believe the answer is:

blanket weed

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'floater on english river' is the definition.
The definition and answer are not the same part of speech.

'floater on english river' is the wordplay.
'floater on' becomes 'blank' (I am not sure about this - if you are sure you should believe this answer much more).
'english' becomes 'E' (abbreviation).
'river' becomes 'Tweed' (river in Scotland).
'blank'+'e'+'tweed'='BLANKET WEED'

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I've seen this clue in The Guardian.

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