Sometimes words make the mind boggle: what on earth, one must wonder upon first encountering it, can a bum-clock be?
Some sort of an aid to achieving regular calls of Nature?
But no, it's not that.
What do you think it is?
No, sorry, not even close.
A bum-clock is a beetle.
In Scotland it's a beetle that makes a droning noise as it flies:
this is a cockchafer or may bug. Photo by Gail Hampshire
but in England bum-clock has been used to mean any insect that hums.
It all makes a bit more sense when you know that bum is another word for hum (and, pleasingly, that something that bums a lot is bumbling...which is, of course, what bumblebees do).
photo by Martin Falbisoner
As for the clock bit...
...sorry, no one's got a clue about that, but the word clock has been used to mean beetle since 1550. A clock-leddy, for instance, is a ladybird.