Poor anagram at start to clue historic document (5,5)
I believe the answer is:
magna carta
'historic document' is the definition.
(I've seen this before)
'poor anagram at start to clue' is the wordplay.
'poor' indicates an anagram.
'start to' indicates taking the first letters.
The first letter of 'clue' is 'c'.
'anagram'+'at'+'c'='anagramatc'
'anagramatc' anagrammed gives 'MAGNA CARTA'.
(Other definitions for magna carta that I've seen before include "Old document" , "Statute of 1215" , "Document signed twelve fifteen" , "historic document" , "Document from 12 15 "given space" .)