Only turning to one instrument (4)
I believe the answer is:
tuba
'instrument' is the definition.
(I know that tuba is a type of bass)
'only turning to one' is the wordplay.
'only' becomes 'but' (but can mean only - eg 'life is but a dream').
'turning' says the letters should be written backwards.
'to one' becomes 'a' (a thing is one thing).
'but' back-to-front is 'tub'.
'tub'+'a'='TUBA'
(Other definitions for tuba that I've seen before include "Brassy thing" , "part of 14 [ORCHESTRA] ?" , "This loud instrument" , "Brass instrument of low pitch" , "Deep brass instrument" .)