Resent heads of exiting nations vacillations year-on-year (4)
I believe the answer is:
envy
'resent' is the definition.
(thesaurus)
'heads of exiting nations vacillations year-on-year' is the wordplay.
'heads of' suggests taking the first letters.
'year-on-year' becomes 'y' (this might be a standard abbreviation I've not previously seen).
The first letters of 'exiting nations vacillations' is 'env'.
'env'+'y'='ENVY'
(Other definitions for envy that I've seen before include "grudging discontent" , "Covet; be jealous of" , "Green-eyed feeling" , "Resentful longing" , "Be green" .)