Such locks are non-ferrous (7)

I believe the answer is:

silvery

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'ferrous' is the definition.
(thesaurus)

I don't understand how the remainder of the clue works.

(Other definitions for silvery that I've seen before include "Having a clear ringing tone; like a precious metal" , "Epithet for the moon" , "Whitish; gently ringing" , "Elementary description of moon" , "Having a white lustrous sheen, like the Spire" .)

I've seen this clue in the Evening Standard.

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