Choo-choo's sound (4)
I believe the answer is:
toot
'choo-choo's sound' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for toot that I've seen before include "Sound one's horn" , "Parp" , "Blow a horn" , "Horn's sound (T, not H)" , "Briefly blow" .)