Horn used in impromptu band (4)
I believe the answer is:
tuba
'horn' is the definition.
(I've seen this before)
'in impromptu band' is the wordplay.
'in' indicates the answer is hidden within the clue.
'TUBA' is hidden in the letters of 'impromptu band'.
'used' acts as a link.
(Other definitions for tuba that I've seen before include "Brass band instrument" , "Large brass wind instrument" , "Stop" , "bit of brass" , "One's blown" .)