One lot of detectives having shower, possibly cause of bad atmosphere (4,4)
I believe the answer is:
acid rain
'cause of bad atmosphere' is the definition.
(acid rain is atmospheric pollution)
'one lot of detectives having shower' is the wordplay.
'one' becomes 'a' (a thing is one thing).
'lot of detectives' becomes 'CID' (Criminal Investigation Department).
'having' is a charade indicator (letters next to each other).
'shower' becomes 'rain' (synonyms).
'a'+'cid'+'rain'='ACID-RAIN'
'possibly' acts as a link.
(Other definitions for acid rain that I've seen before include "Harmful drops" , "Pollution from the sky" , "Damaging precipitation" , "Devastating drop" , "Form of pollution" .)