Drink plugged by prime plug (4,2)
I believe the answer is:
stop up
'plug' is the definition.
(I know that plug can be written as stop up)
'drink plugged by prime' is the wordplay.
'drink' becomes 'sup' (to sup is to drink).
'plugged by' indicates putting letters inside (plug can mean to block up a gap).
'prime' becomes 'top' (both can mean excellent).
'sup' enclosing 'top' is 'STOP UP'.
(Other definitions for stop up that I've seen before include "Don't retire here" , "close" , "Block (eg an earth)" , "Cork" , "Don't go to bed early" .)