Trembling note? (6)
I believe the answer is:
quaver
This clue is a double definition.
'trembling' is the first definition.
(thesaurus)
'note?' is the second definition.
(quaver is a kind of note)
(Other definitions for quaver that I've seen before include "Half-crotchet" , ".Speak in a trembling voice" , "(Of voice) tremble" , "Shake; a short note" , "Tremble - half a crotchet" .)