One enforcing a go-slow emptied water barrel into reservoir (5,4)
I believe the answer is:
speed bump
'one enforcing a go-slow' is the definition.
(speed bumps make drivers go slow)
'emptied water barrel into reservoir' is the wordplay.
'emptied water' becomes 'peed' (water can mean urine).
'barrel' becomes 'b' (industrial abbreviation).
'into' indicates putting letters inside.
'reservoir' becomes 'sump' (sump is a kind of reservoir).
'peed'+'b'='peedb'
'peedb' put within 'sump' is 'SPEED BUMP'.
(Other definitions for speed bump that I've seen before include "Traffic-calming device" , "Traffic-calming measure (B, not H)" , "Sleepng policeman" , "traffic checker" , "Sleeping policeman" .)