Special musical instrument’s pointed (5)
I believe the answer is:
sharp
'pointed' is the definition.
(similar in meaning)
'special musical instrument's' is the wordplay.
'special' becomes 'S' (common abbreviation as in Special Air Service).
'musical instrument' becomes 'harp' (harp is a kind of musical instrument).
's'+'harp'='SHARP'
(Other definitions for sharp that I've seen before include "Clearly defined (image)" , "Keen, knifelike" , "Incisive" , "tart" , "Quick; pointed" .)