Incubator noise (4)
I believe the answer is:
peep
'incubator noise' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for peep that I've seen before include "Peek" , "Furtive look" , "Chirp" , "Sly look (palindrome)" , "Look surreptitiously - through a keyhole?" .)