Essential new wine (4)
I believe the answer is:
must
I believe this clue is a double definition.
'essential' is the first definition.
(I know that must is a type of essential)
'new wine' is the second definition.
(I've seen this before)
(Other definitions for must that I've seen before include "Make off with (plane)" , "Mouldiness" , "Fermenting grape juice" , "Is obliged to" , "Necessity" .)