Knob in brass instrument mentioned (5)
I believe the answer is:
tuber
'knob' is the definition.
(swelling plant)
'brass instrument mentioned' is the wordplay.
'brass instrument' becomes 'tuba' (I've seen this before).
'mentioned' shows a homophone (sound like).
'tuba' is a homophone of 'TUBER'.
'in' is the link.
(Other definitions for tuber that I've seen before include "Brute (anag)" , "or still vegetating?" , "Fleshy underground root" , "A brute of a spud" , "Thickened fleshy root" .)