Reform Act — anagram for historic document (5,5)
I believe the answer is:
magna carta
'historic document' is the definition.
(I've seen this before)
'reform act anagram' is the wordplay.
'reform' indicates anagramming the letters (letters in a new form).
'act'+'anagram'='actanagram'
'actanagram' with letters rearranged gives 'MAGNA CARTA'.
'for' acts as a link.
(Other definitions for magna carta that I've seen before include "1215 charter" , "Statute of 1215" , "Document signed twelve fifteen" , "King John's concession of rights" , "monarch had to accept it" .)