Carbon deposit in earthy matter (4)
I believe the answer is:
clay
'earthy matter' is the definition.
'clay' can be an answer for 'matter' (clay is a kind of matter). I'm unsure of the 'earthy' bit.
'carbon deposit' is the wordplay.
'carbon' becomes 'C' (chemical symbol).
'deposit' becomes 'lay' (I've seen this before).
'c'+'lay'='CLAY'
'in' is the link.
Can you help me to learn more?
(Other definitions for clay that I've seen before include "slip" , "Material used by potter" , "Pigeon target" , "Muhammad Ali's original surname" , "Earth, soil" .)