First lady sick unendingly, which is harmful (4)
I believe the answer is:
evil
'harmful' is the definition.
(synonyms)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for evil that I've seen before include "Morally corrupt" , "Demonic" , "Depravity, wickedness" , "Wicked - sin" , "Morally wrong" .)