Flattened by Jack, perhaps? (6)

I believe the answer is:

planed

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'flattened' is the definition.
Although both the answer and definition are past participle verbs, I don't understand how they can define each other.

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for planed that I've seen before include "having gone up in aircraft" , "Levelled off" .)

I've seen this clue in the Evening Standard.

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