Girl, one after boy from southern Asia (7)
I believe the answer is:
bengali
'from southern asia' is the definition.
'girl one after boy' is the wordplay.
'girl' becomes 'gal' ('gal' can be a synonym of 'girl'**).
'one' becomes 'i' (Roman numeral).
'after' says to put letters next to each other.
'boy' becomes 'ben'.
'gal'+'i'='gali'
'gali' put after 'ben' is 'BENGALI'.
(Other definitions for bengali that I've seen before include "Asian character" , "resident in Calcutta?" , "Asian language" , "Indic language" , "may be Indian" .)