A final note: he introduced last honey and tonic for the mind (6)
I believe the answer is:
psyche
'the mind' is the definition.
('psyche' can be a synonym of 'mind')
'a final note he introduced last honey and tonic' is the wordplay.
I cannot really understand how this works, but
'note' could be 'ps' (a PS or postscript in a letter) and 'ps' is found within the answer.
'he' could be 'che' and 'che' is located in the answer.
A single letter 'y' remains which might be clued in a way I don't see.
This explanation may well be incorrect...
'for' acts as a link.
Can you help me to learn more?
(Other definitions for psyche that I've seen before include "Human mind or spirit" , "Eros's beloved" , "Human soul - Cupid's beloved" , "Wife of Cupid" , "Soul, spirit; lover of Cupid" .)