Excellent sewer sequestering carbon in damaging fluid (4,4)
I believe the answer is:
acid rain
'damaging fluid' is the definition.
(fluid which might do damage)
'excellent sewer sequestering carbon' is the wordplay.
'excellent' becomes 'ai' (resembles 'A1', old-fashioned term for excellent).
'sewer' becomes 'drain' (I've seen this before).
'sequestering' indicates putting letters inside (inserted letters are hidden away or sequestered).
'carbon' becomes 'C' (chemical symbol).
'ai'+'drain'='aidrain'
'aidrain' going around 'c' is 'ACID-RAIN'.
'in' is the link.
(Other definitions for acid rain that I've seen before include "Destructive precipitation" , "What's ecologically damaging" , "Harmful drops" , "pollution falling" , "environmental disaster" .)