Security number: good test for network (4)
I believe the answer is:
ping
'test for network' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
'security number good' is the wordplay.
'security number' becomes 'pin' (a PIN number - personal identification number).
'good' becomes 'g' (abbreviation).
'pin'+'g'='PING'
Can you help me to learn more?
(Other definitions for ping that I've seen before include "Sharp ringing sound" , "Abrupt high-pitched ringing sound" , "Short high-pitched sound" .)