### Clue: Detectives in US agency retired, remaining sharp (6)
### Answer: ACIDIC 
 ### Explanation: Definition: remaining sharp
  CID($$C$$riminal $$I$$nvestigation $$D$$epartment, filled with detectives) **contained in**(in) **reversal of**(retired) CIA(the US $$C$$entral $$I$$ntelligence $$A$$gency)