Scare word (3)
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">scare word</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 boo that I've seen before include "Exclamation to startle" , "Shout of disapproval, maybe at ref" , "Negative reaction" , "Expression of disapproval" , "Cry of disapproval as at villain" .)