Is obliged to (4)
I believe the answer is:
'is obliged to' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for must that I've seen before include "The mould is compelled" , "Unfermented wine" , "The imperative for mould" , "Grape juice ready for fermentation" , "Prerequisite" .)