Sound of percussion instrument is the sign (6)
I believe the answer is:
symbol
'the sign' is the definition.
(symbol is a kind of sign)
'sound of percussion instrument' is the wordplay.
'sound of' shows a homophone (sound like).
'percussion instrument' becomes 'cymbal' (cymbal is a kind of percussion instrument).
'cymbal' is a homophone of 'SYMBOL'.
'is' acts as a link.
(Other definitions for symbol that I've seen before include "Meaningful character" , "Something representative" , "Badge" , "Something that stands for or suggests another thing" , "It signifies" .)