A group of detectives in charge becoming caustic (6)
I believe the answer is:
acidic
'caustic' is the definition.
(thesaurus)
'a group of detectives in charge' is the wordplay.
'group of detectives' becomes 'CID' (Criminal Investigation Department).
'in charge' becomes 'ic' (abbreviation for 'in charge').
'a'+'cid'+'ic'='ACIDIC'
'becoming' acts as a link.
(Other definitions for acidic that I've seen before include "Sarcastic or sour" , "Vinegary" , "Containing a sour-tasting substance" , "Tasting sharp or sour" , "that leaves a bitter taste" .)