Is obliged to (4)
I believe the answer is:
must
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">is obliged to</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(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" .)