Medicine knocked back during vaccinotherapy (5)
I believe the answer is:
tonic
'medicine' is the definition.
(tonic is a kind of medicine)
'knocked back during vaccinotherapy' is the wordplay.
'knocked back' says the letters should be written backwards.
'during' says the answer is hidden in the clue.
'cinot' is hidden in the letters of 'vaccinotherapy'.
'cinot' reversed gives 'TONIC'.
(Other definitions for tonic that I've seen before include "__water; gin mixer" , "Small drink of spirits" , "Invigorating drink" , "Stimulant" , "something medicinal" .)