Vermouth knocked back, sensible drink (6)
I believe the answer is:
tisane
'drink' is the definition.
(tisane is a kind of drink)
'vermouth knocked back sensible' is the wordplay.
'vermouth' becomes 'it' (short for 'Italian vermouth' as in 'gin and it').
'knocked back' is a reversal indicator.
'sensible' becomes 'sane' (I've seen this before).
'it' in reverse letter order is 'ti'.
'ti'+'sane'='TISANE'
(Other definitions for tisane that I've seen before include "Herbal infusion" , "Herbal tea drink" , "brew" , "Herb-tea" , "Herb tea" .)