Small jug found in a drain (5)
I believe the answer is:
sewer
'a drain' is the definition.
(sewer is a kind of drain)
'small jug' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'jug' becomes 'ewer' (type of vessel).
's'+'ewer'='SEWER'
'found in' is the link.
(Other definitions for sewer that I've seen before include "Pipe for carrying away waste matter" , "Drainage conduit" , "It drains" , "Filthy channel" , "Conduit for carrying away waste matter" .)