Be alert! (5,3)
I believe the answer is:
watch out
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">be alert</span>' is the definition.<br/ ><span class="explanation-format__explanation">(thesaurus)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for watch out that I've seen before include "Be vigilant" , "Be careful!" , "this could make synchronisation difficult!" .)