Claudius did murder Hamlet's father by pouring poison in his ears while he slept in his orchard.
However, I am wondering if the clue is a reference to Claudius giving the order to start the duel between Hamlet and Laertes which would end with the death of Claudius, as well the two duellists (and Hamlet's mother who drinks the poisoned drink that Claudius intended for Hamlet, should Laertes's poisoned sword tip fail.)
Claudius gave the order by saying:
"Give me the cups;
And let the kettle to the trumpet speak,
The trumpet to the cannoneer without,
The cannons to the heavens, the heavens to earth,
'Now the king drinks to Hamlet.' Come, begin:
And you, the judges, bear a wary eye."
Is the clue referring to the loud blast of the trumpet?