New wine is essential (4)
I believe the answer is:
must
I believe this clue is a double definition.
'new wine' is the first definition.
(I've seen this in another clue)
'essential' is the second definition.
(I know that must is a type of essential)
'is' is the link.
(Other definitions for must that I've seen before include "Prerequisite" , "The imperative for mould" , "Mould - grape juice during fermentation" , "Make off with (plane)" , "Essential to have" .)