Be vigilant (4,3)
I believe the answer is:
look out
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">be vigilant</span>' is the definition.<br/ ><span class="explanation-format__explanation">(thesaurus)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for look out that I've seen before include "A view of the future" , "Be on your guard!" , "Take care" , "as Wenceslas did" , "Be watchful" .)