Solemn undertaking (6)
I believe the answer is:
pledge
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">solemn undertaking</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for pledge that I've seen before include "Solemn undertaking" , "Deposit" , "Promise - security" , "Plight" , "Promise (to abstain?)" .)