Talk endlessly on carbon and soapstone (4)
I believe the answer is:
talc
'soapstone' is the definition.
(I've seen this in another clue)
'talk endlessly on carbon' is the wordplay.
'endlessly' means to remove the last letter.
'on' says to put letters next to each other.
'carbon' becomes 'C' (C is the chemical symbol for carbon).
'talk' with its final letter taken away is 'tal'.
'tal'+'c'='TALC'
'and' acts as a link.
(Other definitions for talc that I've seen before include "Baby powder" , "Magnesium silicate powder" , "Drier" , "soother" , "Soft mineral; powder" .)