Grasses Sun up! (4)
I believe the answer is:
rats
'grasses' is the definition.
(both can mean to inform on or betray)
'sun up' is the wordplay.
'sun' becomes 'star' (I've seen this before).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'star' back-to-front is 'RATS'.
(Other definitions for rats that I've seen before include "Mild oath - sewer residents" , "Oh no" , "Heels" , "Annoyed expression" , "Damn" .)