Reform Act with anagram for John's charter? (5,5)
I believe the answer is:
magna carta
'john's charter?' is the definition.
(charter agreed by King John in 1215)
'reform act with anagram' is the wordplay.
'reform' is an anagram indicator (letters in a new form).
'with' means one lot of letters go next to another.
'act' after 'anagram' is 'anagramact'.
'anagramact' is an anagram of 'MAGNA CARTA'.
'for' is the link.
(Other definitions for magna carta that I've seen before include "Statute of 1215" , "Old document" , "Important constitutional document" , "Document from 12 15 "given space" , "1215 charter" .)