Child's train (4-4)
I believe the answer is:
choo-choo
'child's train' is the definition.
(childish term for a train)
This is the entire clue.
(Other definitions for choo-choo that I've seen before include "A baby's train" , "A child's train, sounds repetitive" , "Child's word for train" , "Chattanooga train?" , "Baby train?" .)