Doctor cures with bitter pill (5)
I believe the answer is:
curse
'bitter pill' is the definition.
(thesaurus)
'doctor cures' is the wordplay.
'doctor' is an anagram indicator.
'cures' with letters rearranged gives 'CURSE'.
'with' is the link.
(Other definitions for curse that I've seen before include "Invocation of evil; swear" , "Swear - oath" , "Trouble" , "Imprecate" , "Excommunication" .)