British left starts to organise communitarian alliance (4)
I believe the answer is:
bloc
'alliance' is the definition.
(I know that bloc is a type of alliance)
'british left starts to organise communitarian' is the wordplay.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'left' becomes 'l' (common abbreviation).
'starts to' suggests taking the first letters.
The first letters of 'organise communitarian' is 'oc'.
'b'+'l'+'oc'='BLOC'
(Other definitions for bloc that I've seen before include "multinational group" , "League" , "Group of countries with a shared aim" , "Political grouping of countries" , "Axis (political)" .)