What dogs wag (5)
I believe the answer is:
tails
'what dogs wag' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for tails that I've seen before include "Animals' extremities" , "the back of bears - bulls too" , "A possible fall of the pound" , "Coin-tossing call" , "Gambler's choice" .)