Shake (a tail) (3)
I believe the answer is:
wag
'shake a tail' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for wag that I've seen before include "Brisk rear movement or funny fellow" , "Wave to and fro, as a dog's tail" , "Move briskly like dog's tail" , "Vibrate; jester" , "Jocular person" .)