Assassin showing no signs of hesitation set up snare? (4)
I believe the answer is:
drum
'snare?' is the definition.
(thesaurus)
'assassin showing no signs of hesitation set up' is the wordplay.
I cannot quite see how this works, but
'hesitation' could be 'um' and 'um' is located in the answer.
This may be the basis of clue (or it may be nonsense).
(Other definitions for drum that I've seen before include "Thing to be beaten - but not as a container!" , "found in the kitchen?" , "Tap" , "rap" , "(Revolving) cylinder" .)