Liquid waste (4)
(Other definitions for slop that I've seen before include "(Of liquid) spill over" , "overflow" , "Liquid refuse, swill" , "Household liquid refuse" , "discarded > loose garment" .)
'liquid waste' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for effluent that I've seen before include "Liquid industrial waste" , "sewage" , "Waste water" , "It flows and contaminates" , "Sewer water" .)
(Other definitions for urine that I've seen before include "Stuff that's passed" , "Wee" , "Water that one passes" , "Waste product" .)