Poison in medicine's rather elevated (7)
I believe the answer is:
arsenic
'poison' is the definition.
(arsenic can be used as a poison)
'in medicine's rather elevated' is the wordplay.
'in' says the answer is hidden in the clue.
'elevated' says the letters should be written backwards (in a down clue, letters go up).
'cinesra' can be found hidden inside 'medicines rather'.
'cinesra' written backwards gives 'ARSENIC'.
(Other definitions for arsenic that I've seen before include "Poisons in scare" , "Toxic metallic element used as rat poison" , "-- and Old Lace (play)" , "Ratsbane" , "killer" .)