The key note of medicine (5)
I believe the answer is:
tonic
I believe this is a triple definition!
'the key' is the first definition.
(I've seen this before)
'note' is the second definition.
(type of note in a musical scale)
'of' acts as a link.
'medicine' is the third definition.
(I know that tonic is a type of medicine)
(Other definitions for tonic that I've seen before include "Producing tension of muscles" , "Small drink of spirits" , "Quinine drink; keynote" , "Invigorating medicine - keynote" , "requirement for invigoration?" .)