Lasting changes in each key group of counties (4,6)
I believe the answer is:
east anglia
'group of counties' is the definition.
The definition suggests a singular noun which matches the answer.
'lasting changes in each key' is the wordplay.
'changes' is an anagram indicator.
'in' is an insertion indicator.
'each' becomes 'ea' (abbreviation).
'key' becomes 'A' (musical key).
'lasting' is an anagram of 'stangli'.
'ea'+'a'='eaa'
'stangli' put inside 'eaa' is 'EAST ANGLIA'.
Can you help me to learn more?
(Other definitions for east anglia that I've seen before include "Steal again (anag.)" , "part of England" , "Norfolk, Suffolk, part of Essex and Cambridgeshire" , "Area of England" , "English region" .)