Vowelless attention-getter (4)
I believe the answer is:
psst
'vowelless attention-getter' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for psst that I've seen before include "Quiet call for attention" , "Surreptitious attention-seeker" , "Sibilant sound to attract attention furtively" , "This is confidential" , "Whispered attention grabber" .)