Pollutant from a sewer enveloping capital of Caribbean island (4,4)
I believe the answer is:
acid rain
'pollutant' is the definition.
(polluting type of rain)
'a sewer enveloping capital of caribbean island' is the wordplay.
'sewer' becomes 'drain' (I've seen this before).
'enveloping' indicates putting letters inside.
'capital of' indicates taking the first letters.
The first letters of 'caribbean island' is 'ci'.
'a'+'drain'='adrain'
'adrain' placed around 'ci' is 'ACID-RAIN'.
'from' acts as a link.
(Other definitions for acid rain that I've seen before include "Destructive precipitation" , "Harmful precipitation" , "Harmful atmospheric fallout" , "Type of pollution" , "environmental problem" .)