Pollution detected in a Channel Islands sewer (4,4)
I believe the answer is:
acid rain
'pollution' is the definition.
(acid rain is a kind of pollution)
'a channel islands sewer' is the wordplay.
'channel islands' becomes 'CI'.
'sewer' becomes 'drain' (drain can mean a sewer).
'a'+'ci'+'drain'='ACID-RAIN'
'detected in' is the link.
(Other definitions for acid rain that I've seen before include "Polluted water" , "pollution falling" , "Harmful atmospheric fallout" , "Harmful precipitation" , "Type of pollution" .)