Ace detectives finding LSD (4)
I believe the answer is:
acid
'lsd' is the definition.
(synonyms)
'ace detectives' is the wordplay.
'ace' becomes 'a' (abbreviation).
'detectives' becomes 'CID' (abbreviation for Criminal Investigation Department).
'a'+'cid'='ACID'
'finding' acts as a link.
(Other definitions for acid that I've seen before include "Opposite of alkali; sour" , "Sour, sharp-tasting" , "Drug, tart" , "bad rain" , "Biting - tart" .)