Alpine warble (5)
I believe the answer is:
yodel
'alpine warble' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for yodel that I've seen before include "Sing partially falsetto" , "unusual singing" , "Sing partly falsetto" , "mountain air?" , "Sing in high trilling tone like Swiss mountaineer" .)