Historically a knight, in later form, a politician (7)
I believe the answer is:
templar
'historically a knight' is the definition.
I can't judge whether this definition defines the answer.
'in later form a politician' is the wordplay.
'in' is an insertion indicator.
'form' is an anagram indicator.
'a politician' becomes 'mp' (Member of Parliament).
'later' is an anagram of 'telar'.
'telar' enclosing 'mp' is 'TEMPLAR'.
Can you help me to learn more?
(Other definitions for templar that I've seen before include "Member of religious and military order" , "Crusading" , "Old Crusader Knight" , "Simon - (The Saint)" , "barrister" .)