Bracers (6)
I believe the answer is:
'bracers' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for tonics that I've seen before include "Pick-me-ups" , "Medicines or drinks with invigorating effects" , "Potions to strengthen and invigorate" , "contents of medicine cupboards" , "Keys" .)