Be off guard
I believe the answer is:
nap
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">be off guard</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 nap that I've seen before include "Sleep briefly" , "Little sleep" , "Snatch forty winks" , "soft, fluffy surface" , "A brief sleep, forty winks" .)