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Hidden
material
–
timber
under
stone
(5)
Hidden material – timber under stone (5)
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The definition is at the beginning of the clue.
How the clue works
Right, so Hidden material is your definition — a stash of something hidden away. For the wordplay, you want stone abbreviated to ST, then ASH, a type of timber, goes underneath it — ST with ASH tucked under gives you the answer.