Black separatists initially forming gang (4)
I believe the answer is:
jets
'gang' is the definition.
(gang in the musical West Side Story)
'black separatists initially' is the wordplay.
'black' becomes 'jet' (similar in meaning).
'initially' suggests taking the first letters.
The first letter of 'separatists' is 's'.
'jet'+'s'='JETS'
'forming' acts as a link.
(Other definitions for jets that I've seen before include "Aeroplanes - Sharks rivals in "West Side Story"" , "Water spouts or fast planes" , "Rapid streams of liquid or gas" , "Fast airplanes" .)