Much-reduced Eurasian lake, once the fourth-largest inland body of water in the world (4,3)
I believe the answer is:
aral sea
'water in the world' is the definition.
The definition suggests a singular noun which matches the answer.
'much-reduced eurasian lake once the fourth-largest inland body' is the wordplay.
I cannot really see how this works, but
'lake' could be 'aral' (I have seen 'Lake of Asia ' mean 'aral' so perhaps 'lake' could also mean 'aral') and 'aral' is located in the answer.
'body' could be 'sea' (I have seen 'Body of water ' mean 'sea' so perhaps 'body' could also mean 'sea') and 'sea' is found within the answer.
No letters remain.
This explanation may well be incorrect...
'of' acts as a link.
Can you help me to learn more?
(Other definitions for aral sea that I've seen before include "Saltwater lake" , "Shrunken lake" , "Large lake now largely desert" .)