Sound of musical instrument that's swollen (5)
I believe the answer is:
tuber
'that's swollen' is the definition.
I can't tell whether this definition defines the answer.
'sound of musical instrument' is the wordplay.
'sound of' shows a homophone (sound like).
'musical instrument' becomes 'tuba' (tuba is a kind of musical instrument).
'tuba' is a homophone of 'TUBER'.
Can you help me to learn more?
(Other definitions for tuber that I've seen before include "Fleshy underground stem (eg of the potato)" , "or still vegetating?" , "One goes underground" , "Underground stem, root" , "Potato, for example" .)