Ace detectives in charge becoming sour (6)
I believe the answer is:
acidic
'sour' is the definition.
(similar in meaning)
'ace detectives in charge' is the wordplay.
'ace' becomes 'a'.
'detectives' becomes 'CID' (abbreviation for Criminal Investigation Department).
'in charge' becomes 'ic' (abbreviation for 'in charge').
'a'+'cid'+'ic'='ACIDIC'
'becoming' is the link.
(Other definitions for acidic that I've seen before include "that leaves a bitter taste" , "Tasting sharp or sour" , "Sarcastic or sour" , "Sharp-tasting, sour" , "Vinegary" .)