Pipe we included in series (5)
I believe the answer is:
sewer
'pipe' is the definition.
(I know that sewer is a type of drain)
'we included in series' is the wordplay.
'included in' indicates putting letters inside.
'series' becomes 'ser' (abbreviation for series).
'we' placed within 'ser' is 'SEWER'.
(Other definitions for sewer that I've seen before include "Filthy channel" , "Underground waste pipe" , "Disposal outlet" , "Drainage pipe" , "It drains" .)