The top little rivulet makes a vibratory sound (5)
I believe the answer is:
trill
'a vibratory sound' is the definition.
I can't judge whether this defines the answer.
'the top little rivulet' is the wordplay.
'top' says to take the initial letters (in a down clue, the first letter is top).
'little rivulet' becomes 'rill' (synonyms. I am not sure about the 'little' bit.).
The first letter of 'the' is 't'.
't'+'rill'='TRILL'
'makes' is the link.
Can you help me to learn more?
(Other definitions for trill that I've seen before include "Rapid alternation of notes" , "High-pitched warbling sound" , "bit of birdsong" , "Fast alternation of notes" , "sing" .)