Prepare to try gas (not oxygen) when containing bad air (7)
I believe the answer is:
arraign
'prepare to try gas not oxygen' is the definition.
I can't judge whether this definition defines the answer.
'when containing bad air' is the wordplay.
'when' becomes 'argn' (I can't justify this - if you can you should give a lot more credence to this answer).
'containing' is an insertion indicator.
'bad' indicates an anagram.
'air' anagrammed gives 'rai'.
'argn' placed around 'rai' is 'ARRAIGN'.
Can you help me to learn more?
(Other definitions for arraign that I've seen before include "Call before court to answer criminal charge" , "Accuse as in court" , "Accuse before tribunal" , "Accuse publicly" , "Indict, accuse" .)