Catch out (4,2)
I believe the answer is:
trip up
'catch out' is the definition.
(I've seen this before)
This is the entire clue.
(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" .)