Ferment in a jug (4)
I believe the answer is:
stir
This clue is a double definition.
'ferment' is the first definition.
(thesaurus)
'a jug' is the second definition.
(thesaurus)
'in' is the link.
(Other definitions for stir that I've seen before include "jail" , "unrest" , "Mix or agitate" , "can > go forth" , "Agitate; wake from sleep" .)