Needle users found in underground systems (6)
I believe the answer is:
sewers
I believe this is a double definition.
'needle users' is the first definition.
(people who sew using needles)
'underground systems' is the second definition.
(underground systems for sewage waste)
'found in' is the link.
(Other definitions for sewers that I've seen before include "Underground waste pipes" , "Conduits for waste matter" , "drains" .)