The rat posed a menace (6)
I believe the answer is:
threat
'a menace' is the definition.
(I know that menace can be written as threat)
'the rat posed' is the wordplay.
'posed' indicates anagramming the letters (I've seen 'posing' mean this (letters posed in a new order)).
'the'+'rat'='therat'
'therat' is an anagram of 'THREAT'.
(Other definitions for threat that I've seen before include "Hazard" , "An intimidating warning" , "Endangerment" , "Statement of intent to do harm" , "Possibility of danger" .)