Drink -- a suitable one agent knocked back (8)
I believe the answer is:
aperitif
'drink' is the definition.
(aperitif is a kind of drink)
'a suitable one agent knocked back' is the wordplay.
'suitable' becomes 'fit' (similar in meaning).
'one' becomes 'i' (Roman numeral).
'agent' becomes 'rep' (short for representative).
'knocked back' is a reversal indicator.
'fit'+'i'+'rep'='fitirep'
'fitirep' back-to-front is 'peritif'.
'a'+'peritif'='APERITIF'
(Other definitions for aperitif that I've seen before include "Appetising drink" , "Alcoholic drink taken before meal" , "Alcoholic beverage as an appetiser" , "Drink stimulating the appetite" , "An alcoholic drink before dinner, an appetiser" .)