Dismisses instrument, by the sound of it (5)
I believe the answer is:
sacks
'dismisses' is the definition.
('sack' can be a synonym of 'dismiss')
'instrument by the sound of it' is the wordplay.
'instrument' becomes 'sax' (saxophone).
'by the sound of it' shows a homophone (sound like).
'sax' sounds like 'SACKS'.
(Other definitions for sacks that I've seen before include "Cashiers" , "Fires; large bags" , "Large strong bags" , "Plunders; fires" , "Gets rid of" .)