It always comes first (6)
I believe the answer is:
prefix
This could be a double definition.
'it always comes' is the first definition.
The definition suggests a singular noun which matches the answer.
'first' is the second definition.
The definition and answer can be both related to communication as well as being singular nouns.
Perhaps you can see an association between them that I don't see?
Can you help me to learn more?
(Other definitions for prefix that I've seen before include "put at the front" , "Eg, pre-, inter-" , "Word placed before another to change its meaning" , "Meaningful construct attached to the beginning of a word" , "Word added in front of another" .)