Sweet thing in pub with poisonous atmosphere? (4,3)

'poisonous atmosphere?' is the definition.
The definition suggests a singular noun which matches the answer.

'sweet thing in pub' is the wordplay.
I cannot really understand how this works, but
'sweet' could be 'mars' (brand of chocolate bar) and 'mars' is present in the answer.
'pub' could be 'bar' (both are drinking establishments) and 'bar' is found in the answer.
No letters remain.
This explanation may well be incorrect...

'with' acts as a link.

Can you help me to learn more?