Spy's tweet (4)
I believe the answer is:
peep
I believe this clue is a double definition.
'spy's' is the first definition.
(I've seen this before)
'tweet' is the second definition.
(thesaurus)
(Other definitions for peep that I've seen before include "Look secretly" , "Spy" , "Large underground chamber" , "Chirp" , "Look surreptitiously - through a keyhole?" .)