Dangerous WWI agent really good with chat (7,3)
I believe the answer is:
mustard gas
'dangerous wwi agent' is the definition.
I can't judge whether this definition defines the answer.
'really good with chat' is the wordplay.
'really good' becomes 'mustard' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'with' means one lot of letters go next to another.
'chat' becomes 'gas' (informal term meaning to chat).
'mustard'+'gas'='MUSTARD GAS'
Can you help me to learn more?
(Other definitions for mustard gas that I've seen before include "Blistering vapour from a poisonous liquid" , "Irritant used in chemical weapons" , "Killer" , "Chemical warfare agent" .)