Opening of hundred sewers, not supplying North and South? That’s negligent (8)
I believe the answer is:
heedless
'that's negligent' is the definition.
(thesaurus)
'opening of hundred sewers not' is the wordplay.
'opening of hundred sewers' becomes 'heed' (I can't explain this - if you can you should believe this answer much more).
'not' becomes 'less' (I've seen this in other clues).
'heed'+'less'='HEEDLESS'
'supplying north and south?' is the link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for heedless that I've seen before include "Rash" , "Without caution" , "Inattentive, unmindful" , "Taking no notice" , "Imprudent" .)