Place that's fouled by waters in the nether regions? (4)
I believe the answer is:
styx
'waters in the nether regions?' is the definition.
The definition suggests a singular noun which matches the answer.
'place that's fouled by' is the wordplay.
I cannot really understand how this works, but
'place' could be 'sty' (I have seen 'Filthy place ' mean 'sty' so perhaps 'place' could also mean 'sty') and 'sty' is located in the answer.
'by' could be 'x' (2 by 4 is 2 x 4) and 'x' is present in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for styx that I've seen before include "Hellish current" , "The last river one crosses" , "River surrounding Hades" , "Underground river" , "River Charon rowed on" .)