Perhaps an Iranian, is nowhere near by metric system (5)
I believe the answer is:
farsi
'system' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for farsi that I've seen before include "Modern Persian lang." , "Language of Iran" , "Persian language" , "Iran's official language" , "Iranian language, also widely spoken in Afghanistan" .)