Unusual sea-plant is so agreeable (8)

I believe the answer is:

pleasant

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'so agreeable' is the definition.
'pleasant' can be an answer for 'agreeable' (similar in meaning). I'm not certain of the 'so' bit.

'unusual sea-plant' is the wordplay.
'unusual' is an anagram indicator.
'sea'+'plant'='seaplant'
'seaplant' is an anagram of 'PLEASANT'.

'is' is the link.

Can you help me to learn more?

(Other definitions for pleasant that I've seen before include "sweet?" , "A greeable, affable" , "Delightful" , "Refreshing" , "Enjoyable, nice" .)

I've seen this clue in the Sydney Morning Herald.

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