Part of agreement is a necessary medical infusion (6)
I believe the answer is:
tisane
'infusion' is the definition.
(I've seen this before)
'part of agreement is a necessary medical' is the wordplay.
'part of' indicates the answer is hidden within the clue.
'TISANE' is hidden in the letters of 'agreement is a necessary medical'.
(Other definitions for tisane that I've seen before include "Herbal infusion" , "relaxing drink" , "Herb tea" , "Infusion of e.g. dried or fresh flowers or leaves" , "Herbal tea drink" .)