Sign language (4)
I believe the answer is:
logo
This clue could be a double definition.
'sign' is the first definition.
(logo is a kind of sign)
'language' 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 can't see?
Can you help me to learn more?
(Other definitions for logo that I've seen before include "Small emblem or device" , "Symbolic design" , "Corporate emblem" , "A firm's sign or symbol" , "Printed symbol as organisation's emblem" .)