He runs the fingerprint through a police database and discovers a matching fingerprint. This is a "hit" as opposed to a "miss" where there is no match found.
The Clapper was a device sold by television ads 10 or 2o years ago that purported to assist you in turning lamps and other devices on and off by the simple action of you make a sharp sound twice in a row -- clapping.
It was marketed and perceived as a device for the elderly -- this is what is meant in the text you cite. The prune juice is also a reference to old age.