Signal impatience: throw out a parasitical plant (9)

I believe the answer is:

toothwort

'a parasitical plant' is the definition.
(type of parasitical plant)

'signal impatience throw out' is the wordplay.
'signal impatience' becomes 'toot' (as in tooting a car horn).
'out' is an anagram indicator (out can mean wrong or inaccurate).
'throw' is an anagram of 'hwort'.
'toot'+'hwort'='TOOTHWORT'

(Another definition for toothwort that I've seen is " plant".)

I've seen this clue in The Times.

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