Solemn pledge (3)
I believe the answer is:
vow
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">solemn pledge</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for vow that I've seen before include "Promise, swear" , "Solemn pledge" , "Binding promise" , "Oath or solemn promise" , "Pledge, take oath" .)