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Some elements are in the back with Thorium (4,5)

Ross

I believe the answer is:

rare earth

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(Other definitions for rare earth that I've seen before include "metallic element" , "Oxide" , "One of a particular group of metallic elements" , "One of a group of 15 hard to distinguish elements - at her rear (anag)" , "element such as promethium" .)

I've seen this clue in The Hindu.
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