Menace from the rat (6)
I believe the answer is:
threat
'menace' is the definition.
(I know that menace can be written as threat)
'from the rat' is the wordplay.
'from' indicates anagramming the letters (the anagram comes from the same letters).
'the'+'rat'='therat'
'therat' with letters rearranged gives 'THREAT'.
(Other definitions for threat that I've seen before include "A warning of harm to come" , "An intimidatory warning" , "Impending danger" , "Statement of intention to harm" , "Menacing words" .)