Calm about rat’s tail being turned into fertiliser (9)

I believe the answer is:

composted

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'turned into fertiliser' is the definition.
I can't tell whether this definition defines the answer.

'calm about rat's tail being' is the wordplay.
'calm about rat's tail' becomes 'compos' (I can't justify this - if you can you should give a lot more credence to this answer).
'being' becomes 'Ted' (Ted is an example).
'compos'+'ted'='COMPOSTED'

Can you help me to learn more?

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

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