A boy's introduction to slide whistle (4)
I believe the answer is:
hiss
'whistle' is the definition.
(thesaurus)
'a boy's introduction to slide' is the wordplay.
I cannot really see how this works, but
'boy' could be 'hi' and 'hi' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for hiss that I've seen before include "Boo" , "Sound of serpent" , "(Make noise to) express disapproval" , "Sibilant sound made through the teeth" , "Sharp sibilant sound" .)