Poisonous vapour good with arsenic? (3)
I believe the answer is:
gas
'poisonous vapour' is the definition.
'gas' can be an answer for 'vapour' (I've seen this before). I'm not certain of the 'poisonous' bit.
'good with arsenic?' is the wordplay.
'good' becomes 'g' (abbreviation).
'with' says to put letters next to each other.
'arsenic?' becomes 'As' (As is the chemical symbol for arsenic).
'g'+'as'='GAS'
Can you help me to learn more?
(Other definitions for gas that I've seen before include "Oxygen is one" , "Eg, hydrogen" , "rabbit" , "person who's amusing ..." , "Fumes, vapour" .)