The keynote to good health? (5)
I believe the answer is:
tonic
'the keynote' is the definition.
(synonyms)
'to good health?' is the wordplay.
I cannot really see how this works, but
'to' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for tonic that I've seen before include "requirement for invigoration?" , "something providing a boost" , "Stimulant" , "Small drink of spirits" , "Accompaniment to gin?" .)