Catch out (4,2)
I believe the answer is:
trip up
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">catch out</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for trip up that I've seen before include "Cause person to stumble, or detect in error" , "Go wrong" , "'Cause to stumble (4,2)'" , "Blunder" , "fluff" .)