Evil spirits disturbed Slough (6)
I believe the answer is:
ghouls
'evil spirits' is the definition.
(ghoul is a kind of evil spirit)
'disturbed slough' is the wordplay.
'disturbed' indicates anagramming the letters.
'slough' with letters rearranged gives 'GHOULS'.
(Other definitions for ghouls that I've seen before include "Evil spirits that rob graves and eat corpses" , "Mythical creatures that devour corpses" , "Malevolent spirits" , "Grave-robbing spirits found in slough" , "Evil spirits or people interested in morbid things" .)