United Nations bar not closed (4)
I believe the answer is:
bloc
'united nations' is the definition.
(I've seen this before)
'bar not closed' is the wordplay.
'bar' becomes 'block' (synonyms).
'not closed' means to remove the last letter.
'block' with its final letter taken away is 'BLOC'.
(Other definitions for bloc that I've seen before include "Alliance - coalition" , "Political alliance" , "united nations" , "Political or economic confederation" , "Group with a common policy" .)