Lab rat's challenge
I believe the answer is:
maze
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">lab rat's challenge</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 maze that I've seen before include "Confusing network of paths" , "Labyrinth, tangle" , "confusion" , "Puzzling network of paths" , "Complex network" .)