Be suspicious, fixing small tear (5,1,3)
I believe the answer is:
smell a rat
'be suspicious' is the definition.
(I've seen this before)
'fixing small tear' is the wordplay.
'fixing' indicates anagramming the letters.
'small'+'tear'='smalltear'
'smalltear' is an anagram of 'SMELL A RAT'.
(Other definitions for smell a rat that I've seen before include "Have suspicion something is afoot" , "Detect something suspicious" , "Suspect something wrong" , "Find something fishy" , "Sense something suspicious" .)