Mistakenly call the malignant hag (7)
I believe the answer is:
hellcat
'malignant hag' is the definition.
(I've seen this before)
'mistakenly call the' is the wordplay.
'mistakenly' indicates an anagram.
'call'+'the'='callthe'
'callthe' with letters rearranged gives 'HELLCAT'.
(Other definitions for hellcat that I've seen before include "she-devil" , "Spiteful woman" , "enemy of Cerberus?" , "Unpleasant female" , "Witch" .)