So a plant’s unexpectedly situated behind a hooter (9)

I believe the answer is:

postnasal

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'situated behind a hooter' is the definition.
I can't tell whether this definition defines the answer.

'so a plant's unexpectedly' is the wordplay.
'unexpectedly' is an anagram indicator.
'so'+'a'+'plants'='soaplants'
'soaplants' is an anagram of 'POSTNASAL'.

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I've seen this clue in The Times.

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