More detective work!

In cryptic puzzles, spies and detectives crop up quite a lot. SPIES or SECRET AGENTS or SECURITY SERVICES often clues the letter combo CIA (Central Intelligence Agency in the US, obvs). DETECTIVES, meanwhile, will often clue the combo CID (Criminal Investigation Department of the British police), but DETECTIVE can also clue PI (private investigator). DETECTIVE INSPECTOR = DI, DETECTIVE SERGEANT = DS