Soft white limestone (5)
I believe the answer is:
chalk
'soft white limestone' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for chalk that I've seen before include "Calcium carbonate" , "Soft white rock used for writing on boards etc" , "Opposite of cheese?" , "Substance totally different from cheese!" , "Blackboard writing implement" .)