Briefly obstruct alliance (4)
I believe the answer is:
bloc
'alliance' is the definition.
(bloc is a kind of alliance)
'briefly obstruct' is the wordplay.
'briefly' means to remove the last letter.
'obstruct' becomes 'block' (blocking is a kind of obstructing).
'block' with its final letter removed is 'BLOC'.
(Other definitions for bloc that I've seen before include "Group of countries in special alliance" , "Political allies" , "Allied country group" , "Political or economic confederation" , "Political group of nations" .)