Alec's out of Muscatels and freshly pressed juice (4)
I believe the answer is:
must
'pressed juice' is the definition.
The answer and definition are not the same part of speech. However, past participle verbs and adjectives sometimes mean the same thing.
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for must that I've seen before include "Fermenting grape juice" , "Prerequisite" , "Unfermented wine" , "Mould - grape juice during fermentation" , "The mould is compelled" .)