Exclamation used to attract someone’s attention surreptitiously (4)

I believe the answer is:

psst

(Other definitions for psst that I've seen before include "Quiet attention-seeker" , "Noise used to attract someone's attention privately" , "Pay attention" , "(Whispered) may I have your attention?" , "that attracts attention" .)

I've seen this clue in the Sydney Morning Herald.

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