A head of America proclaimed truce (6)
I believe the answer is:
apiece
'a head' is the definition.
(I have seen 'A head' mean 'apiece' so perhaps 'head' could also mean 'apiece')
'america proclaimed truce' is the wordplay.
'america' becomes 'a' (common abbreviation eg in USA).
'proclaimed' shows a homophone (sound like).
'truce' becomes 'peace'.
'peace' sounds like 'piece'.
'a'+'piece'='APIECE'
'of' is the link.
(Other definitions for apiece that I've seen before include "Per person" , "For or by each one of group" , "for everyone" , "A head" .)