Primate gets religious instruction having knocked back suitable drink (8)
I believe the answer is:
aperitif
'drink' is the definition.
(aperitif is a kind of drink)
'primate gets religious instruction having knocked back suitable' is the wordplay.
'primate' becomes 'ape' (ape is a kind of primate).
'gets' says to put letters next to each other.
'religious instruction' becomes 'RI' (abbreviation).
'having knocked back' is a reversal indicator.
'suitable' becomes 'fit' (similar in meaning).
'fit' back-to-front is 'tif'.
'ape'+'ri'+'tif'='APERITIF'
(Other definitions for aperitif that I've seen before include "it's served before dinner" , "What's drunk before a meal?" , "Drink stimulating the appetite" , "If pirate may have a drink before dinner?" , "alcohol before meal" .)