Sound of tapping metal? (4)
I believe the answer is:
ping
'sound' is the definition.
(ping is a kind of sound)
'of tapping metal?' is the wordplay.
'of' says the answer is hidden in the clue.
'PING' can be found hidden inside 'tapping al'.
(Other definitions for ping that I've seen before include "Sharp ringing sound" , "Short high-pitched sound" , "Abrupt high-pitched ringing sound" .)