Natural divider is semi-moulded about gazebo’s base (7)

I believe the answer is:

meiosis

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'natural divider' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.

'is semi-moulded about gazebo's base' is the wordplay.
'moulded' indicates anagramming the letters.
'about' indicates putting letters inside.
'base' says to take the final letters (the base of the word).
The final letter of 'gazebo' is 'o'.
'is'+'semi'='issemi'
'issemi' is an anagram of 'meisis'.
'meisis' placed around 'o' is 'MEIOSIS'.

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

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