But a broken instrument (4)
I believe the answer is:
tuba
'instrument' is the definition.
(tuba is a kind of instrument)
'but a broken' is the wordplay.
'broken' indicates anagramming the letters.
'but'+'a'='buta'
'buta' is an anagram of 'TUBA'.
(Other definitions for tuba that I've seen before include "A bass instrument" , "Brass band instrument" , "horn" , "Valved instrument" , "One's blown" .)