Warble beginning of tune by stream (5)
I believe the answer is:
trill
'warble' is the definition.
(synonyms)
'beginning of tune by stream' is the wordplay.
'beginning of' suggests taking the first letters.
'by' says to put letters next to each other.
'stream' becomes 'rill' (rill is a kind of stream).
The first letter of 'tune' is 't'.
't'+'rill'='TRILL'
(Other definitions for trill that I've seen before include "Fast alternation of notes" , "Quavering note" , "Quavering or vibratory sound" , "High-pitched warbling sound" , "Sound of rapidly alternating notes" .)