Take an oath (5)
I believe the answer is:
swear
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">take an oath</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 swear that I've seen before include "Affirm" , "Use bad language" , "Eff and blind" , "Assert" , "Promise on oath" .)