Carbon-rich partially rotted deposit of vegetable matter (4)
I believe the answer is:
peat
This clue might be a double definition.
'carbon' is the first definition.
(thesaurus)
'vegetable matter' is the second definition.
(I've seen this in another clue)
'rich partially rotted deposit of' acts as a link.
I am not very happy about this link. Some or all of it may belong to one of the definitions.
Can you help me to learn more?
(Other definitions for peat that I've seen before include "Some got energy from this" , "Vegetable deposit used as fuel and fertiliser" , "Turf as in bogland" , "Dried turf" , "Dried plant material used as fuel" .)