Tell on someone (6)
I believe the answer is:
tattle
'tell on someone' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for tattle that I've seen before include "Tell tales" , "Chatter idly" , "'Gossip, idle talk (6)'" , "Divulge secrets" , "Inform on someone" .)