Tell (on) (3)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
rat
'tell on' is the definition.
(thesaurus)
This is all the clue.
(Other definitions for rat that I've seen before include "Tell-tale" , "Contemptible person" , "Long-tailed rodent" , "Treacherous rodent" , "Renegade" .)