Surreptitious summons (4)
I believe the answer is:
psst
'surreptitious summons' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for psst that I've seen before include "Soft sound to get someone's attention" , "Whispered exclamation to attract attention" , "Hissing noise to attract attention" , "Sibilant interjection to call attention" , "attempt to make secret contact" .)