The term 'Hamelin Vermin' refers to the rats in the Brothers Grimm fairy tale, 'The Pied Piper of Hamelin'. The story tells of a piper who was hired by the town of Hamelin to rid it of its RAT infestation. The piper succeeded in his task, but when the town refused to pay him, he used his magical pipe to lure away the children of the town. The crossword solution for this term is 'RAT', which is a three-letter word.