Part of the world, mostly unchanged area (4)
I believe the answer is:
asia
'part of the world' is the definition.
(I've seen this before)
'mostly unchanged area' is the wordplay.
'mostly' means to remove the last letter.
'unchanged' becomes 'asis' (I've seen this before).
'area' becomes 'a' (maths abbreviation).
'asis' with its final letter removed is 'asi'.
'asi'+'a'='ASIA'
(Other definitions for asia that I've seen before include "Populous area" , "A great deal of land" , "The Orient" , "Continent bordering three oceans" , "space for economic expansion?" .)