Each country’s restricting the onset of major emissions (10)

'emissions' is the definition.
('emanation' can be a synonym of 'emission')

'each country's restricting the onset of major' is the wordplay.
'each' becomes 'ea' (abbreviation).
'country's' becomes 'nations' ('nation' can be a synonym of 'country').
'restricting' means one lot of letters goes inside another.
'the onset of' says to take the initial letters.
The initial letter of 'major' is 'm'.
'ea'+'nations'='eanations'
'eanations' going around 'm' is 'EMANATIONS'.