It tends to delay the reader (5)
I believe the answer is:
comma
This clue could be a double definition.
'it tends to delay' is the first definition.
The definition suggests a singular noun which matches the answer.
'reader' is the second definition.
The answer and definition can be both related to communication as well as being singular nouns.
Perhaps you can see a link between them that I can't see?
'the' acts as a link.
Can you help me to learn more?
(Other definitions for comma that I've seen before include "Kind of butterfly" , "Common punctuation park" , "Punctuation mark - butterfly" , "Brief stop" , "List-item separator" .)