Leaders of harsh extremists in central region hang together (6)
I believe the answer is:
cohere
'hang together' is the definition.
(thesaurus)
'leaders of harsh extremists in central region' is the wordplay.
'leaders of' suggests taking the first letters.
'in' means one lot of letters goes inside another.
'central region' becomes 'core' (**).
The initial letters of 'harsh extremists' is 'he'.
'he' put into 'core' is 'COHERE'.
(Other definitions for cohere that I've seen before include "Stick or hold together and resist separation" , "Stick firmly together" , "Cling" , "Fit together" , "Form a whole" .)