State invaded by English party retaliating (7)
I believe the answer is:
avenger
'party retaliating' is the definition.
(I've seen this before)
'state invaded by english' is the wordplay.
'state' becomes 'aver' (both can mean to declare).
'invaded by' is an insertion indicator.
'english' becomes 'eng' (abbreviation).
'aver' placed around 'eng' is 'AVENGER'.
(Other definitions for avenger that I've seen before include "person wanting justice through violence?" , "Inflicter of retribution" , "One carries out retribution" , "one seeking to get even" , "Retaliator" .)