Crossword Genius

A hard bed (7)

Ross

I believe the answer is:

stratum

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

I can't explain the remainder of the clue.

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(Other definitions for stratum that I've seen before include "Bed of sedimentary rock" , "Layer (of rock)" , "rock band" , "(Geological) layer" , "stretch of rock" .)

I've seen this clue in the Evening Standard.
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