Something vital (4)
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">something vital</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(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" .)