New to being employed by the UN (6)
I believe the answer is:
unused
'new to' is the definition.
(thesaurus)
'employed by the un' is the wordplay.
'employed' becomes 'used' ('use' can be a synonym of 'employ').
'by' is a charade indicator (letters next to each other).
'used' put after 'un' is 'UNUSED'.
'being' acts as a link.
(Other definitions for unused that I've seen before include "Pristine" , "Surplus" , "Not accustomed to" , "Left over" , "hadn't been exploited" .)