Drunk on port? At once! (6)
I believe the answer is:
pronto
'at once' is the definition.
(thesaurus)
'drunk on port?' is the wordplay.
'drunk' is an anagram indicator.
'on'+'port'='onport'
'onport' with letters rearranged gives 'PRONTO'.
(Other definitions for pronto that I've seen before include "Right now, amigo" , "Proton (anag.)" , "Quickly, without delay" , "Quickly (sl.)" , "Right now, immediately" .)