Get rid of only an old instrument (7)
I believe the answer is:
sackbut
'old instrument' is the definition.
(I've seen this before)
'get rid of only' is the wordplay.
'get rid of' becomes 'sack' (as in sacking a worker).
'only' becomes 'but' (but can mean only - eg 'life is but a dream').
'sack'+'but'='SACKBUT'
'an' is the link.
(Other definitions for sackbut that I've seen before include "Musical instrument of the Renaissance" , "Old instrument" , "one blows into it" , "Ancient musical instrument" , "music maker" .)