Downpour taking top off sewer (4)
I believe the answer is:
rain
'downpour' is the definition.
(pouring rain)
'taking top off sewer' is the wordplay.
'taking top off' means to remove the first letter.
'sewer' becomes 'drain' (I've seen this before).
'drain' with its initial letter removed is 'RAIN'.
(Other definitions for rain that I've seen before include "See 12" , "Come down in showers" , "The ---, it ---eth every day (T.Night)" , "Fall wetly" , "Falling moisture" .)