Something vital
I believe the answer is:
must
'something vital' 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 "Fermenting grape juice" , "Mould - grape juice during fermentation" , "Grape juice ready for fermentation" , "Make off with (plane)" , "The mould is compelled" .)