Party chief’s territory (6)
I believe the answer is:
domain
'territory' is the definition.
(I've seen this in another clue)
'party chief's' is the wordplay.
'party' becomes 'do' (do is a kind of party).
'chief' becomes 'main' (synonyms).
'do'+'main'='DOMAIN'
(Other definitions for domain that I've seen before include "Territory over which rule or control is exercised" , "Range of influence or field of activity" , "Realm or range of influence" , "Territory over which someone reigns" , "A district sphere of influence" .)