Half opting twice for train (4-4)
I believe the answer is:
choo-choo
'train' is the definition.
(I've seen this before)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for choo-choo that I've seen before include "Child's railway train" , "Baby train?" , "(Childish) train" , "A baby's train" , "A child's train, sounds repetitive" .)