Greenhouse gas, CO2 (6,7)
I believe the answer is:
carbon dioxide
I believe this clue is a double definition.
'greenhouse gas' is the first definition.
(I've seen this before)
'co2' is the second definition.
(I've seen this before)
(Other definitions for carbon dioxide that I've seen before include "Gas produced in burning" , "Gas in the atmosphere" , "This one might expire" , "Gas for dry ice" , "Gas produced in breathing" .)