Charity concerned with carbon in waste (3,5)
I believe the answer is:
red cross
'charity' is the definition.
'concerned with carbon in waste' is the wordplay.
'concerned with' becomes 're' (re can mean regarding, concerning).
'carbon' becomes 'C' (chemical symbol).
'in' is an insertion indicator.
'waste' becomes 'dross' (dross is a kind of waste).
'c' going within 'dross' is 'dcross'.
're'+'dcross'='RED-CROSS'
(Other definitions for red cross that I've seen before include "Humanitarian outfit" , "that brings relief!" , "International organisation for care of wounded in war" , "International organization that cares for victims of war" , "International organisation for treatment of wounded in war" .)