Brief: retain for murder cases (6)
I believe the answer is:
Here is my best explanation:
'brief' is the definition.
(I've seen this before)
'retain for murder cases' is the subsidiary indication.
'cases' indicates the answer is hidden within the clue.
'inform' can be found hidden inside 'retain for murder'.
(Other definitions for inform that I've seen before include "Impart knowledge", "Tell - shop", "Notify or tell", "Give knowledge to", "Brief", "Give information to (someone)", "Pass on knowledge", "Apprise: grass!", "Certify" and "snitch?".)