Whoops! Stop including whispered exclamation (4)
I believe the answer is:
psst
'whispered exclamation' is the definition.
(I've seen this before)
'whoops stop including' is the wordplay.
'including' indicates the answer is hidden within the clue.
'PSST' is hidden in the letters of 'whoops stop'.
(Other definitions for psst that I've seen before include "Soft sound to get someone's attention" , "Quiet attention-seeker" , "attempt to make secret contact" , "that attracts attention" , "way of attracting attention" .)