Humble United Nations doofus fuming after losing leadership (10)
I believe the answer is:
unassuming
'humble' is the definition.
(thesaurus)
'united nations doofus fuming after losing leadership' is the wordplay.
'united nations' becomes 'UN' (abbreviation).
'doofus' becomes 'ass' (I've seen this in another clue).
'after losing leadership' means to remove the first letter.
'fuming' with its initial letter removed is 'uming'.
'un'+'ass'+'uming'='UNASSUMING'
(Other definitions for unassuming that I've seen before include "Modest" , "not taking things for granted" , "disrobing?" , "Not pretentious" .)