Crown jewels (5)

I believe the answer is:

tiara

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'crown' is the definition.
(I've seen this before)

I cannot understand how the remainder of the clue works.

(Other definitions for tiara that I've seen before include "Jewelled head ornament" , "Jewelled coronet" , "Woman's crescent-shaped ornamental headdress" , "pope may have been topped by one" , "Jewelled, ornamental headgear" .)

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

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