Pollutant in a Channel Islands sewer? (4,4)
I believe the answer is:
acid rain
'pollutant' is the definition.
(polluting type of rain)
'a channel islands sewer?' is the wordplay.
'channel islands' becomes 'CI'.
'sewer?' becomes 'drain' (sewer is a kind of drain).
'a'+'ci'+'drain'='ACID-RAIN'
'in' is the link.
(Other definitions for acid rain that I've seen before include "Form of pollution" , "Pollution from the sky" , "Type of pollution" , "Harmful atmospheric fallout" , "Destructive precipitation" .)