Part of pilot's speech rejected - it attracts attention (4)
I believe the answer is:
psst
'it attracts attention' is the definition.
(sound used to attract someone's attention)
'part of pilot's speech rejected' is the wordplay.
'part of' says the answer is hidden in the clue.
'rejected' shows that the letters should be reversed in order.
'tssp' is hidden within 'pilots speech'.
'tssp' in reverse letter order is 'PSST'.
(Other definitions for psst that I've seen before include "Don't tell anyone" , "This is confidential" , "Sound used to attract someone's attention" , "Here's a secret" , "Sound to call quietly" .)