Clue: Talking of setter's wheezing heartlessly creates an pleasant atmosphere (6)
Answer: MIASMA
Explanation: Definition: an pleasant atmosphere
Sounds like ('talking of') MY('setters') plus AS[th]MA ('wheezing') minus the middle letters ('heartlessly'). Why the article 'an'? Because a miasma is an unpleasant atmosphere – the UN went AWOL at some stage.