Become suspicious (5,1,3)
I believe the answer is:
smell a rat
'become suspicious' is the definition.
(I've seen this before)
This is the entire clue.
(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" .)