Hat's on instrument (7)
I believe the answer is:
leghorn
'hat's' is the definition.
(leghorn is a kind of hat)
'on instrument' is the wordplay.
'on' becomes 'leg' (cricket term, fielding positions).
'instrument' becomes 'horn' (horn is a kind of instrument).
'leg'+'horn'='LEGHORN'
(Other definitions for leghorn that I've seen before include "Breed of domestic foul" , "Chicken; straw hat" , "port" , "Breed of chicken" .)