Nice if hat fits head (9)
I believe the answer is:
chieftain
'head' is the definition.
(chieftain is a kind of head)
'nice if hat fits' is the wordplay.
'fits' indicates an anagram.
'nice'+'if'+'hat'='niceifhat'
'niceifhat' with letters rearranged gives 'CHIEFTAIN'.
(Other definitions for chieftain that I've seen before include "Tribe's leader" , "Clan leader" , "Tribal leader" , "Head of a clan" , "Tribal chief" .)