Horse carrying half of blue stone (6)
I believe the answer is:
marble
'stone' is the definition.
(I know that marble is a type of stone)
'horse carrying half of blue' is the wordplay.
'horse' becomes 'mare' (mare is a kind of horse).
'carrying' means one lot of letters goes inside another.
'half of' suggests halving the letters.
'blue' with half the letters taken is 'bl'.
'mare' enclosing 'bl' is 'MARBLE'.
(Other definitions for marble that I've seen before include "Polished rock used in sculpture" , "Crystalline limestone often used in sculpture" , "Alley possibly" , "Decorative stone - ball" , "Crystallised limestone" .)