The old sound of typing? (4)
I believe the answer is:
ping
'the old sound' is the definition.
'ping' can be an answer for 'sound' (ping is a kind of sound). I'm not certain of the 'the old' bit.
'typing?' is the wordplay.
'PING' is hidden within the letters.
But, I am uncertain how the hidden word is indicated.
'of' acts as a link.
Can you help me to learn more?
(Other definitions for ping that I've seen before include "Abrupt high-pitched ringing sound" , "Sharp ringing sound" , "Short high-pitched sound" .)