Block socialists' first summit (4)
I believe the answer is:
stop
'block' is the definition.
(I know that block can be written as stop)
'socialists' first summit' is the wordplay.
'socialists' first' becomes 's' (1st letter of 'socialists').
'summit' becomes 'top' (synonyms).
's'+'top'='STOP'
(Other definitions for stop that I've seen before include "period in US" , "Standstill" , "Finished" , "Pack it in - bar" , "cut out" .)