Instrument from arrangement of baritone music? Not so (10)
I believe the answer is:
tambourine
'instrument' is the definition.
(tambourine is a kind of instrument)
'arrangement of baritone music? not so' is the wordplay.
'arrangement of' indicates anagramming the letters.
'not' indicates named letters should be taken away.
'so' becomes 'sic' ('so' in Latin).
'baritone'+'music'='baritonemusic'
'baritonemusic' with 'sic' taken out is 'baritonemu'.
'baritonemu' is an anagram of 'TAMBOURINE'.
'from' acts as a link.
(Other definitions for tambourine that I've seen before include "Shaken instrument" , "One uses jingles" , "Percussion instrument" , "drum" .)