Drain small jug (5)
I believe the answer is:
sewer
'drain' is the definition.
(I know that sewer is a type of drain)
'small jug' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'jug' becomes 'ewer' (type of vessel).
's'+'ewer'='SEWER'
(Other definitions for sewer that I've seen before include "It drains" , "One working with material?" , "Drainage conduit" , "Filthy channel" , "Underground drain" .)