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 wordplay.
'cases' indicates the answer is hidden within the clue.
'INFORM' is hidden in the letters of '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?".)