Clue: Cause alarm, as I've done with Du Tompes? (3,3,3,5,3,7)
Answer: PUT THE CAT AMONG THE PIGEONS
Explanation: Definition: Cause alarm
'dupe'='pigeon', someone who is the victim of a deception 
"Du Tompes" is 'Tom'=CAT, put inside/among 'Dupes'=PIGEONS
 
…I initially thought SET instead of PUT