Soft silvery-white metallic element whose atomic number is 56 (6)

I believe the answer is:

barium

(Other definitions for barium that I've seen before include "Umbria (anag.)" , "Metallic element, Ba" , "Content of meal taken before X-ray" , "Substance swallowed to facilitate X-ray" .)

I've seen this clue in The Independent.

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