Perhaps one's going to darn filthy place (5)
I believe the answer is:
sewer
'filthy place' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'perhaps one's going to darn' is the wordplay.
'perhaps one' becomes 'er' (I can't explain this - if you can you should give a lot more credence to this answer).
'going to' says to put letters next to each other.
'darn' becomes 'sew' (I've seen this in another clue).
'er' put after 'sew' is 'SEWER'.
Can you help me to learn more?
(Other definitions for sewer that I've seen before include "Drain - seamstress?" , "Large pipe for carrying waste matter" , "Tailor" , "Filthy channel" , "Disposal outlet" .)