Torment a nocturnal mammal (6)
I believe the answer is:
badger
This clue is a double definition.
'torment' is the first definition.
(badgering is a kind of tormenting)
'a nocturnal mammal' is the second definition.
(I've seen this before)
(Other definitions for badger that I've seen before include "A mammal; harry" , "Black and white animal called Brock" , "protected species" , "One of the weasel family" , "Black and white burrowing mammal" .)