Stone grave best covered up (7)
I believe the answer is:
peridot
'stone' is the definition.
(type of precious stone)
'grave best covered up' is the wordplay.
'grave' becomes 'dire' (I've seen this in another clue).
'best' becomes 'top' (top can mean the very best).
'covered' indicates putting letters inside.
'up' is a reversal indicator (in down clue: letters go upwards).
'dire' inserted into 'top' is 'todirep'.
'todirep' reversed gives 'PERIDOT'.
(Other definitions for peridot that I've seen before include "mineral" , "Gem" , "olivine" , "Top ride (anag) - gemstone" , "I deport (anag) - gemstone" .)