Poor grade E established (6)

I believe the answer is:

agreed

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'established' is the definition.
Both the answer and definition are adjectives. Maybe you can see a link between them that I don't see?

'poor grade e' is the wordplay.
'poor' is an anagram indicator.
'grade'+'e'='gradee'
'gradee' with letters rearranged gives 'AGREED'.

Can you help me to learn more?

(Other definitions for agreed that I've seen before include "Consented and concurred" , "In harmony" , "Acceded" , "matched" , "Tallied - reconciled" .)

I've seen this clue in The Times.

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