Clue: Monster bar ends being trashed, of course (12)
Answer: BANDERSNATCH
Explanation: Definition: Monster
(BAR ENDS)* + NATCH ("of course"). The Bandersnatch made its first appearance in *Jabberwocky* – "Beware the Jabberwock, my son!/The jaws that bite, the claws that catch!/Beware the Jubjub bird, and shun/The frumious Bandersnatch!" – and is also mentioned in *The Hunting of the Snark