Obsolete lines that run east and south (5)
I believe the answer is:
runes
'obsolete lines' is the definition.
The definition and answer can be both related to communication as well as being plural nouns.
Maybe you can see an association between them that I can't see?
'run east and south' is the wordplay.
'east' becomes 'E' (synonyms).
'and' means one lot of letters go next to another.
'south' becomes 'S' (abbreviation).
'run'+'e'+'s'='RUNES'
'that' acts as a link.
Can you help me to learn more?
(Other definitions for runes that I've seen before include "Old letters" , "Mysterious letters" , "old characters" , "marks on old tablets" , "Small stones bearing symbols with magic significance" .)