Poison found in medicine's rapidly returned (7)
I believe the answer is:
arsenic
'poison' is the definition.
(thesaurus)
'in medicine's rapidly returned' is the wordplay.
'in' indicates a hidden word.
'returned' says the letters should be written backwards.
'cinesra' can be found hidden inside 'medicines rapidly'.
'cinesra' back-to-front is 'ARSENIC'.
'found' acts as a link.
(Other definitions for arsenic that I've seen before include "Can rise result from poison?" , "-- and Old Lace (play)" , "killer" , "Element with many poisonous compounds" , "Poison used in pesticides and weedkillers" .)