Subtle summons (4)
I believe the answer is:
psst
'subtle 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 "Surreptitious attention-seeker" , "Whispered exclamation to attract attention" , "(Whispered) may I have your attention?" , "Whispered attention grabber" , "Sound of call to hear a secret" .)