Forestall reduced group of countries (4)
I believe the answer is:
bloc
'group of countries' is the definition.
(group of associated nations, eg the Eastern bloc)
'forestall reduced' is the wordplay.
'forestall' becomes 'block' (blocking is a kind of forestalling).
'reduced' means to remove the last letter.
'block' with its last letter taken away is 'BLOC'.
(Other definitions for bloc that I've seen before include "Political group of nations" , "People cooperating" , "Political or economic confederation" , "-- Forsyth, entertainer" , "Group with a common policy" .)