A southern island area where most people live (4)
I believe the answer is:
asia
'where most people live' is the definition.
(I've seen this in another clue)
'a southern island area' is the wordplay.
'southern' becomes 's' (abbreviation).
'island' becomes 'i' (geographical abbreviation).
'area' becomes 'a' (maths abbreviation).
'a'+'s'+'i'+'a'='ASIA'
(Other definitions for asia that I've seen before include "Put out" , "The nations of the East, collectively" , "group of countries" , "The Orient" , "Huge landmass" .)