Set aside key with hard outer casing? (7)

I believe the answer is:

rescind

'set aside' is the definition.
(thesaurus)

I cannot understand how the rest of the clue works.

(Other definitions for rescind that I've seen before include "Set aside law" , "Countermand" , "Revoke (order)" , "Quash" , "Discern in order to cancel, say a law" .)

I've seen this clue in The Times.

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