It tends to delay the reader (5)
I believe the answer is:
comma
This might 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 they are linked in a way I don't understand?
'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" .)