Turkey is part of this region in retreat (4,4)
I believe the answer is:
near east
'turkey is part of this' is the definition.
The definition suggests a singular noun which matches the answer.
'region in retreat' is the wordplay.
'region' becomes 'area' (synonyms).
'in' means one lot of letters goes inside another.
'retreat' becomes 'nest' (nest is a kind of retreat).
'area' placed within 'nest' is 'NEAR EAST'.
Can you help me to learn more?
(Other definitions for near east that I've seen before include "group of countries" , "Region between India and the Mediterranean" , "Iran and Iraq, etc" , "The Balkans and Turkey" , "where communities are often in conflict" .)