It indicates I will be missing from 'Who's Who' (10)
I believe the answer is:
apostrophe
This clue could be a double definition.
'it indicates i' is the first definition.
The definition suggests a singular noun which matches the answer.
'will be missing from who's who' is the second definition.
The answer and definition can be both related to communication as well as being singular nouns.
Maybe there's an association between them I don't understand?
Can you help me to learn more?
(Other definitions for apostrophe that I've seen before include "Sign of omission" , "At which point discarded letters" , "Single quote mark" , "Passage addressed to character" , "in Charley's Aunt ?" .)