Knock it back — healthy drink (6)
I believe the answer is:
tisane
'drink' is the definition.
(tisane is a kind of drink)
'knock it back healthy' is the wordplay.
'knock' shows that the letters should be reversed in order (I've seen 'knocking' mean this).
'back healthy' becomes 'sane' (sane can mean mentally healthy. I am not sure about the 'back' bit.).
'it' back-to-front is 'ti'.
'ti'+'sane'='TISANE'
(Other definitions for tisane that I've seen before include "brew" , "Infusion of e.g. dried or fresh flowers or leaves" , "Herb-tea" , "Herb tea" , "Infusion (eg of herbs or flowers)" .)