Answer:
The key here that could help the student infer the definition of "sedative" is to take a look at the consequences of the revelation. It can be understood that if this revelation turns out to be "sedative", it will produce the Draught Stout a feeling of "relief", as he states "I feel a new man, I'd have lain awake worrying about that murder all night." So, if the revelation is "sedative", it will calm down the Draught Stout consternation for the case.