Declares about English people getting their own back? (8)
I believe the answer is:
avengers
'people getting their own back?' is the definition.
'declares about english' is the wordplay.
'declares' becomes 'avers' (averring is a kind of declaring).
'about' is an insertion indicator.
'english' becomes 'eng'.
'avers' enclosing 'eng' is 'AVENGERS'.
(Other definitions for avengers that I've seen before include "They retaliate" , "Retribution takers" , "1960s TV series with Steed and Emma Peel" , "Retaliating fighters" , "Agents of retribution" .)