Sculptor's medium (6)
(Other definitions for bronze that I've seen before include "Get a tan" , "Hard alloy suitable for casting" , "Third prize" , "Copper and tin alloy - sculpture" , "Popular alloy for sculptures" .)
'sculptor's medium' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for marble that I've seen before include "Small ball of glass used in games" , "Crystallised limestone" , "Hard shiny stone used by sculptors" , "that won't yield, possibly an > ally" , "Hard crystalline limestone used for sculpture" .)