Old gunslinger sharpens up hearing aid (9)
I believe the answer is:
earphones
'hearing aid' is the definition.
(earphones let you hear things)
'old gunslinger sharpens up' is the wordplay.
'old gunslinger' becomes 'Earp' (19th-century US lawman Wyatt Earp).
'sharpens up' becomes 'hones' (as in honing a blade).
'earp'+'hones'='EARPHONES'
(Other definitions for earphones that I've seen before include "listening devices" , "Listening device" , "One phrase (anag.)" .)