Become suspicious (5,1,3)
I believe the answer is:
smell a rat
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">become suspicious</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 smell a rat that I've seen before include "Suspect trickery (colloq.)" , "Find something fishy" , "Suspect something wrong" , "Have suspicion something is afoot" , "Sense something suspicious" .)