This bone is under the thumb (9)

I believe the answer is:

trapezium

(Other definitions for trapezium that I've seen before include "Quadrilateral with a parallel pair of sides" , "Certainly no square" , "Quadrilateral - small wrist bone" , "Quadrilateral with pairs of unequal parallel sides" , "Geometric shape" .)

I've seen this clue in The Times.

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