Form of pollution -- a constant one with sewer (4,4)
I believe the answer is:
acid rain
'form of pollution' is the definition.
(I've seen this before)
'a constant one with sewer' is the wordplay.
'constant' becomes 'c' (common symbol in maths for an unknown constant).
'one' becomes 'i' (Roman numeral).
'with' is a charade indicator (letters next to each other).
'sewer' becomes 'drain' (drain can mean a sewer).
'a'+'c'+'i'+'drain'='ACID-RAIN'
(Other definitions for acid rain that I've seen before include "environmental problem" , "Devastating drop" , "Dangerous precipitation" , "Polluted precipitation" , "Harmful drops" .)