It takes your breath away (6)
'it takes your breath away' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for asthma that I've seen before include "Mat has a breathing disorder" , "which leaves one breathless" , "Chronic breathing condition" , "Shortage of breath" , "Mat has difficult breathing" .)
(Other definitions for carbon dioxide that I've seen before include "gas from exhaust?" , "Gas breathed out" , "Gas produced in burning" , "Gas formed by breathing out" , "Gas produced in breathing" .)