Medicine upsets my deer (6)
I believe the answer is:
remedy
'medicine' is the definition.
(I know that remedy is a type of medicine)
'upsets my deer' is the wordplay.
'upsets' indicates anagramming the letters.
'my'+'deer'='mydeer'
'mydeer' with letters rearranged gives 'REMEDY'.
(Other definitions for remedy that I've seen before include "solution" , "A cure, for disease or other ill" , "Nostrum" , "answer" , "Put right, be a cure for" .)