Early form of trombone (7)
I believe the answer is:
sackbut
'early form of trombone' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for sackbut that I've seen before include "music maker" , "Old instrument" , "antique instrument" , "Early form of trombone" , "Medieval form of trombone" .)