Extensive letters prove the borders of England have been fixed for centuries (4-11)
I believe the answer is:
long-established
'been fixed for centuries' is the definition.
The answer and definition are different parts of speech. However, past participle verbs and adjectives occasionally mean the same thing.
'extensive letters prove the borders of england' is the wordplay.
'extensive letters' becomes 'long' (I can't explain this - if you can you should give a lot more credence to this answer).
'prove' becomes 'establish' (synonyms).
'the borders of' says to hollow out the word (remove centre letters).
'england' with its middle removed is 'ed'.
'long'+'establish'+'ed'='LONG-ESTABLISHED'
'have' acts as a link.
Can you help me to learn more?