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 quite see 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 present in the answer.
'by' could be 'x' (2 by 4 is 2 x 4) and 'x' is found in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
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" .)