Tonic going backwards and forwards (7)
I believe the answer is:
reviver
'tonic' is the definition.
(I've seen this before)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for reviver that I've seen before include "Restorer to life" , "Tonic" , "One who resuscitates" , "Pick-me-up" .)