North-south route across southern Australian land mass (4)
I believe the answer is:
asia
'land mass' is the definition.
(I've seen this before)
'north-south route across southern australian' is the wordplay.
'north-south route' becomes 'ai' (resembles A1, UK north-south road).
'across' indicates putting letters inside.
'southern' becomes 's' (abbreviation).
'australian' becomes 'a'.
'ai' placed around 's' is 'asi'.
'asi'+'a'='ASIA'
(Other definitions for asia that I've seen before include "Major land mass" , "huge territory" , "Thailand and its location" , "Stretch of land" , "It has most of the earth's population" .)