Cricket sound
I think it could be either:
'cricket sound' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for chirp that I've seen before include "Tweet" , "Sound made by small bird" , "Twitter" , "Short sharp tone of grasshopper" , "Fledgling's sound" .)
(Another definition for chirr that I've seen is " trill".)