Syllable of disapproval (3)
'syllable of disapproval' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for tut that I've seen before include "Express disapproval" , "Disapproving comment" , "Annoyed exclamation" , "in short I don't like that!" , "Sound of mild disapproval" .)