Prevent legally
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">prevent legally</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 estop that I've seen before include "Bar, for a lawyer" , "Hinder, preclude (law)" , "Legally check" , "Legally prevent" , "Legal term" .)