Im going to run my first homebrew dnd campaign and need help with the plot hook. I have taken a lot of time creating a fun world to play in with interestig places but havent pit alot of time in the acctuall plot hook wich will bring then into these cool places. Here is my idea a friend of the party has been taken by city guards on suspicion of murder but as this being a good friend and loyal person to the party they belief he surely wouldnt kill someone so when they see him about to be publicly executed they will hopefully intervene. A city guard will listen to what they will say and basically say they have 3 days to find the 'real' killer or they will kill the partys friend. I have no clue what to do from here i belief i should give a very simple clue but want the players to find it themselfs so what kind of clue could i give. The setting is a spelljammer city with a radien citadel kind of atmosphere.

TLDR: I need help giving a clue to find a murderer in a spelljammer radient citadel setting.