Cesspool after draining inside oozed out (8)
I believe the answer is:
excluded
'cesspool after draining' is the definition.
I can't judge whether this defines the answer.
'inside oozed out' is the wordplay.
'inside' becomes 'cl' (I can't explain this - if you can you should give a lot more credence to this answer).
'oozed' becomes 'exuded' ('exude' can be a synonym of 'ooze').
'out' is an insertion indicator.
'cl' going within 'exuded' is 'EXCLUDED'.
Can you help me to learn more?
(Other definitions for excluded that I've seen before include "Left out" , "Denied access" , "'Debarred, shut out (8)'" , "Ruled out" , "Kept out from place or group" .)