Nasty smell when oxygen is inhaled — it's lethal (6,3)
I believe the answer is:
poison gas
'it's lethal' is the definition.
'nasty smell when oxygen is inhaled' is the wordplay.
'nasty smell' becomes 'pong' (I've seen this before).
'when' becomes 'as'.
'oxygen' becomes 'o' (atomic symbol for oxygen).
'inhaled' means one lot of letters goes inside another.
'o'+'is'='ois'
'pong'+'as'='pongas'
'pongas' going around 'ois' is 'POISON-GAS'.
(Other definitions for poison gas that I've seen before include "Such as chlorine" , "Eg, sarin or phosgene" .)