An outside number that's unidentified (4)
I believe the answer is:
anon
'number that's unidentified' is the definition.
I can't judge whether this defines the answer.
'an outside' is the wordplay.
'an' becomes 'a'.
'outside' becomes 'non'.
'a'+'non'='ANON'
Can you help me to learn more?
(Other definitions for anon that I've seen before include "At another time, of old" , "Soon - nobody knows me" , "Having no known name" , "Shortly; unknown" , "soon to be written" .)