Dogs -- they could be wagging (5)
I believe the answer is:
tails
I believe this is a double definition.
'dogs' is the first definition.
(dog and tail can both mean to follow)
'they could be wagging' is the second definition.
(a dog's tail wags)
(Other definitions for tails that I've seen before include "Coin-tossing call" , "Gambler's choice" , "as opposed to heads" , "Ends" , "Morning coat" .)