Newly-pressed grape juice is essential (4)
I believe the answer is:
must
'essential' is the definition.
(similar in meaning)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for must that I've seen before include "Is obliged to" , "Mouldiness" , "Necessity" , "Essential to have" , "Prerequisite" .)