Confidential warning (3-3)
I believe the answer is:
tip-off
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">confidential warning</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for tip-off that I've seen before include "Give advance warning or hint" , "Alert" , "Informed hint" , "Piece of secret information" , "Confidential warning" .)