Be suspicious and sense a traitor (5,1,3)
I believe the answer is:
smell a rat
'be suspicious' is the definition.
(I've seen this in another clue)
'sense a traitor' is the wordplay.
'sense' becomes 'smell' (smell is a kind of sense).
'traitor' becomes 'rat' (rat can mean a treacherous person).
'smell'+'a'+'rat'='SMELL A RAT'
'and' is the link.
(Other definitions for smell a rat that I've seen before include "Detect something suspicious" , "Suspect something's amiss" , "Suspect trickery (colloq.)" , "Suspect something wrong" , "Find something fishy" .)