Tell someone's alter ego from yours at first sight (6)
I believe the answer is:
notify
'tell' is the definition.
(thesaurus)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for notify that I've seen before include "Declare" , "Formally tell" , "Formally advise" , "No one" , "Advise, apprise" .)