Reason to withdraw permit to fly (6)

I believe the answer is:

ground

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'reason' is the definition.
(synonyms)

I can't explain the rest of the clue.

(Other definitions for ground that I've seen before include "territory" , "cause" , "Land - crushed to powder" , "Solid part of the earth's surface" , "on which Pachelbel wrote his Canon?" .)

I've seen this clue in The Globe and Mail.

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