A hard bed (7)
I believe the answer is:
stratum
'bed' is the definition.
(I've seen this before)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for stratum that I've seen before include "Bed of sedimentary rock" , "Layer (of rock)" , "rock band" , "(Geological) layer" , "stretch of rock" .)