Horse carries pound back in stone (6)
I believe the answer is:
marble
'stone' is the definition.
(marble is a kind of stone)
'horse carries pound back' is the wordplay.
'horse' becomes 'mare' (mare is a kind of horse).
'carries' means one lot of letters goes inside another (inserted letters carried inside).
'pound' becomes 'lb' (abbreviation for the unit of weight).
'back' says the letters should be written backwards.
'lb' in reverse letter order is 'bl'.
'mare' enclosing 'bl' is 'MARBLE'.
'in' is the link.
(Other definitions for marble that I've seen before include "Glass ball; stone" , "'Hard, crystalline rock (6)'" , "Hard shiny stone used by sculptors" , "Hard crystalline limestone, can be polished" , "Rock used for sculptures" .)