Little dog running up and down (3)
I believe the answer is:
pup
This clue is a double definition.
'little dog' is the first definition.
(short for puppy)
'running up and down' is the second definition.
(Other definitions for pup that I've seen before include "Whelp" , "Canine young" , "Young of canines" , "Produce litter" , "Small palindromic dog" .)