Just a line, not a complete letter (5)
I believe the answer is:
serif
'letter' is the definition.
The answer and definition can be both related to communication as well as being singular nouns.
Maybe you can see an association between them that I can't see?
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for serif that I've seen before include "Decorative part of font" , "Projection on a letter in some fonts" , "Short line at the end of the main strokes of a printed character" , "Feature in some fonts" , "Little bit of character" .)