Form of Roman law, not a western standard (6)
I believe the answer is:
normal
'a western standard' is the definition.
'normal' can be an answer for 'standard' (I've seen this in another clue). I'm not sure about the 'a western' bit.
'form of roman law not' is the wordplay.
'form of' indicates an anagram (the letters need to be in another 'form').
'law not' becomes 'l' (I am not sure about this - if you are sure you should believe this answer much more).
'roman' with letters rearranged gives 'norma'.
'norma'+'l'='NORMAL'
Can you help me to learn more?
(Other definitions for normal that I've seen before include "Ordinary, standard" , "'Usual, standard (6)'" , "Standard - well-adjusted" , "Unexceptional" , "Standard, typical" .)