Sign for the input (5)
I believe the answer is:
caret
This clue could be a double definition.
'sign' is the first definition.
(caret is a kind of sign)
'the input' is the second definition.
The definition and answer can be both related to communication as well as being singular nouns.
Maybe you can see a link between them that I don't see?
'for' acts as a link.
Can you help me to learn more?
(Other definitions for caret that I've seen before include "this reveals something missing" , "Upward arrow mark" , "Symbol to show something is to be inserted" , "Sign something's lacking" , "Indicator of omission" .)