Toxic gas from behind engulfing almost all of pub (6)

'toxic gas' is the definition.
'arsine' can be an answer for 'gas' (I have seen 'Poisonous gas' mean 'arsine' so perhaps 'gas' could also mean 'arsine'). I'm not certain of the 'toxic' bit.

'behind engulfing almost all of pub' is the wordplay.
'behind' becomes 'arse' (synonyms).
'engulfing' means one lot of letters goes inside another.
'almost all of' means to remove the last letter (I've seen 'almost all' mean this).
'pub' becomes 'inn' (I've seen this before).
'inn' with its last letter taken off is 'in'.
'arse' placed around 'in' is 'ARSINE'.

'from' is the link.

Can you help me to learn more?