A note is needed for medicine (5)
I believe the answer is:
tonic
This clue could be a double definition.
'a note' is the first definition.
(type of note in a musical scale)
'needed for medicine' is the second definition.
'tonic' can be an answer for 'medicine' (tonic is a kind of medicine). I'm not sure about the 'needed for' bit.
'is' is the link.
Can you help me to learn more?
(Other definitions for tonic that I've seen before include "requirement for invigoration?" , "Quinine drink; keynote" , "something providing a boost" , "Invigorating medicine - keynote" , "Producing tension of muscles" .)