Asian language, second on distant island (5)
'asian language' is the definition.
(language of Iran)
'second on distant island' is the wordplay.
'second' becomes 's' (common abbreviation).
'on' means one lot of letters go next to another.
'distant' becomes 'far' (similar in meaning).
'island' becomes 'i'.
's' after 'far' is 'fars'.
(Other definitions for farsi that I've seen before include "Iranian language, also widely spoken in Afghanistan", "Language of Iran", "Persian language", "Iran's official language", "foreign language".)